Preparation And Characterization Of Anion Exchange Membranes Based On Imidazolium For Alkaline Fuel Cells

As a promising energy conversion device,fuel cells are of growing academic and practical interest due to their superior energy density,slight chemical emissions and compact systems.Of various types of fuel cells,proton exchange membrane fuel cells（PEMFCs）,which operate in acidic enviornment,have been explored.However,superfluous material cost originating from both the requisite noble metal catalyst and Nafion membrane electrolytes has retarded its practical application.In contrast,anion exchange membrane fuel cells（AEMFCs）are envisioned as a desirable alternative to PEMFCs due to the faster electrochemical reaction kinetics in alkaline condition and the advantage of empolying non-noble catalysts.Nevertheless,the preparation of anion exchange membranes（AEMs）that could possess both high ionic conductivity and adequate stability remains the inherent challenge for the AEMFCs.In this thesis,three series of imidazolium based anion exchange membranes were designed and synthesized for alkaline fuel cells,including interpenetrating network membrane（IPNs）,densely flexible side chains type membrane and clustered multication side chains type membrane.Their structure-morphology-property relationship was systematically studied.Besides,their praticall application was also investigated in the H₂/O₂ alkaline single fuel cell systems.First of all,a series of interpenetrating AEMs was prepared via the crosslinking of poly（1-vinylimidazole）and poly（vinylbenzyl chloride）.The formed imidazolium cations acts as bifunctional groups for both crosslinking sites and OH-transport.The addition of hydrophilic poly（vinyl alcohol）endowed the membranes with improved mechanical properties.The ionic conductivity and dimensional stability increased with the crosslinking density.Notably,PP-50 membrane with ionic exchange content（IEC）of 1.86 meq·g^-1 has a swelling ratio and OH^- conductivity of 37.5%and 41.4 mS·cm^-1 at 60 ℃,respectively.With the intention of further improving the ionic conductivity,a novel kind of densely flexible side chains type AEMs was synthesized by grafting bromine-bearing imidazolium-based ionic liquids into four hydroxyl-bearing poly（ether sulfone）matrix.This newly polymer structure combines the advantages of flexible side chains and densely local ionic concentration structure.As a consequence,the distinguished hydrophilic/hydrophobic phase-separated morphology and nano-channels were formed within the materials.Therefore,an enhancement in the ion conductivity were achieved.The AEMs have OH-conductivity in excess of 110 mS·cm^-1 at 80 ℃,which is much higher than traditional AEM materials.Moreover,the water uptake and swelling ratio were controlled in the range of 14.6%-44.3%and 15.3%-33.3%,respectively.The superb single cell output also suggests its promise for fuel cells.Furthermore,by designing the side chain structure and the reaction condition,a multi-imidazolium cation side chains type AEMs were prepared successfully.A single long side chain was charged for the first time with three imidazolium cations,thus the polymer main chains could obtain lessened grafting ration under similar IEC,consequently enhanced hydrophobicity.Together with the flexible and hydrophilic alkyl side chains,the AEMs demonstrated excellent self-assembly capability and the ionic clusters tended to aggregate to hundreds of nanometers.Owing to the favorable microphase morphology,the AEMs showed a conductivity of 120.5 mS·cm^-1 at 80 ℃,and the normalized conductivity was also up to 3.73 mS·cm^-1,which indicates an excellent ionic transport efficiency.Moreover,the swelling ratio of TrimPES-0.2 was 12%at 30 ℃ because of the improved backbone hydrophobicity.This also endows the AEMs with good stability in alkaline solution.A peak power density of 126 mW·cm^-2 was achieved for the TrimPES-0.4 signel cell.Therefore,these findings suggest that by tuning the distribution of cations,AEMs with well-balanced performance could be reached for fuel cell applications.