Hydroxide Ion Conductive Channel Engineering in Aryl and Norbornene-based Anion Exchange Membranes

Author:Chen Wan Zuo

Supervisor:wu xue mei he gao hong paul akohl


Degree Year:2019





Anion exchange membrane fuel cells(AEMFCs)have attracted much attention due to the outstanding advantages.such as fuel flexibility and non-precious metal electrocatalysts.As the core component of AEMFCs.anion exchange membranes(AEMs)have been investigated and several notable advances have been made in recent years,however,alkaline stability and hydroxide conductivity still need to be improved for the commercialization of AEMs.Ion conductive channels are believed to be an effective way to enhance the alkaline stability and hydroxide conductivity.In this work,mechanism of aggregation of cationic groups is proposed from the aspect of the electrostatic forces between anions and cations by molecular dynamics simulation.Then inorganic salt is used as a soft template to regulate the electrostatic forces in AEMs,constructing the ion channels.At last,the designs of multi-cationic side chains and all-hydrocarbon backbones lead to the ion channels with different patterns and sizes,achieving the excellent alkaline stability and high hydroxide conductivity.A novel alkaline group.HMTA,has been employed as mono-quaternization reagent to prepare HMTA mono-quaternized polysulfone anion exchange membranes via a heterogeneous quaternization route.By analyzing the microstructures and interactions in HMTA quaternized polysulfone membranes via molecular dynamics simulations,it is found that the electrostatic forces between anions and cations result in the aggregation of cationic groups.The interactions between ion pairs enhance the interactions between polymer chains,improving the anti-swelling,thermal and alkaline stabilities.As a result,the size of ionic clusters in PSF-QuOH is about d-6.7 nm;at 60 ℃,PSF-QuOH(IEC=2.23 mmol g-1)has the conductivity of 72.4 mS cm-1 and swelling ratio of 21.0%.Based on the aggregation mechanism of ionic clusters.,Zn2+ ion is proposed as a soft template and incorporated in imidazolium-functionalized polysulfone membranes,regulating the electrostatic forces in AEMs and constructing the ion conductive channels in the induced membranes.By a combined experimental data and molecular dynamics simulations,a novel two-step ionic aggregation method,induced by electrostatic attraction of cation additive(Zn2+),has been proposed.In the first step,Zn2+ions,imidazolium(Im+)groups,Cl-and CH3COO-are found to be aggregated together to form Zn2+clusters(~50 nm)in the incorporated membranes(PSF-ImCl/Zn-x).The optimal amount of Zn2 should provide the moderate size of the Zn2+ionic network and correlate the majority of Im+ groups,which has the potential to induce a better Im+ aggregation after the removal of Zn2+ ions.The content of Zn2+ ions has a great influence on the compactness,water sorption and hydroxide conductivity of the induced PSF-ImOH-x membranes.Additionally,based on a porous anodic aluminium oxide template,AEMs with well-ordered arrays of ion channels are prepared and achieve a remarkable two-fold enhancement of the through-plane conductivity over that of a traditional homogeneous membrane.The tri-quaternized side chain is designed based on DABCO to induce the ion conductive channels in poly(ether sulfone)membranes.Di-cationic quaternary ammonium salt is synthesized and reacts with chloromethylated poly(ether sulfone)to form the third cation on the side chain.By this way.three cations could be introduced into each side chain.strengthen ionic interactions and enhance micro-phase separation.The multi-cationic and flexible side chains increase local concentration of cationic groups and the mobility of the cationic groups,which enhance interaction among ionic domains and promote aggregation of ionic clusters.Tri-QPESOH membranes achieve a well-balanced performance between high hydroxide conductivity and dimensional stability.The ionic clusters in the Tri-QPESOH are~13.4 um.The Tri-QPESOH membrane with IEC 2.3 1 mmol g-1 exhibits high hydroxide conductivity of 130.9 mS cm-1 at 80 ℃,and low swelling ratio of 21.5%even at 80 ℃.The diblock copolymers and homopolymers with all-hydrocarbon backbones based on poly(norbomene)are proposed to improve the ion conductive channels and the alkaline stability of the polymer backbones with aryl ether.Poly(bromopropyl norbomene)homopolymers and poly(bromopropyl norbomene)-block-poly(butyl norbomene)diblock copolymers are systhesized by ring opening rmetathesis polymerization(ROMP)of norbornene monomers and subsequent hydrogenation,achieving the ion channels with the ionic clusters and lamellar morphology.N,N,N,N’-tetramethyl-1,6-hexanediamine(TMHDA)is used as a cross-linking agent to control water uptake and mechanical instability.XL20-rPNB-LY100 homopolymer AEM(4.51 mmol g-1)exhibits the obvious ionic clusters(25.4 nm),highest conductivity of 194.8 mS cm-1 at 80 ℃ and excellent alkaline stability with no detectable degradation in conductivity after 792 h in 1 M NaOH at 80 ℃.