Structure Configuration And Performance Regulation Of Anion Exchange Membranes For Alkaline Fuel Cells

Polymer electrolyte membrane fuel cells,as new-style energy conversion devices,has drawn tremendous attention in recent years due to their advantages of high conversion efficiency,quick start-up rate,wide ffuel sources,pollution-free and simple moving and dismounting.According to the working principle,fuel cells can be divided into proton exchange membrane fuel cell(PEMFC)and alkaline anion exchange membrane fuel cell(AAEMFC).Ion exchange membranes,as the important components,have seen rapid development.However,perfluorosulfonic acid membrane such as Nafion is still the only commercial proton exchange membrane(PEM)for PEMFC.Moreover,the noble metal platinum is necessary as the catalyst.While incorporation of nonprecious metal catalysts such as Ag,Ni and higher enhanced oxygen reduction reaction kinetics for AAEMFC in comparison with PEMFC has attracted more attention and development for anion exchange membrane(AEM).However,compared to PEM,AEM has lower hydroxide conductivity,worse mechanical and chemical stability,which has limited its development.It is found that regulating the molecular structure of ionomer is vital for membrane properties in addition to altering functional groups.The initial and simplest molecular structure is the main-chain type one,whose synthesis is very simple.While the resulting main-chain type membrane exhibited poor performance.Inspired by the specific structure with hydrophobic main chain and hydrophilic side chain of Nafion,side-chain type and comb-shaped AEMs were prepared.Improved ionic conductivity,water resistance and alkaline stability were obtained for these membranes.Furthermore,increasing the density of functional groups in side chains can further improve membrane properties.The research described above indicates that regulating the molecular structure of ionomers is an effective solution for improving membrane properties.However,there is still no systematic investigation of ionomer structure and the molecular structures are limited to main-chain type and side-chain type.We therefore intend to introduce the molecular structure from polymer science into AEM to bring better properties.The detailed contents are summarized as follows:1)To obtain side-chain type AEM,alkynyl-azide click reaction was utilized to graft side chain with alkyl chain pendant quaternary ammonium(QA)center to bromomethylated poly(phenylene oxide)(BPPO)main chain.Triazole groups are therefore resulted between main chains and QA groups.This method is facile and highly controllable.The obtained side-chain typed AEM exhibited high ionic conductivity and low water swelling.Its hydroxide conductivity is up to 140mS/cm at 80 ?,while the corresponding water uptake is as low as 16.5wt%.The good performance of AEM is benefited from the side-chain type structure with triazole groups and hydrophobic alkyl chains on side chains.The method used here is versatile for membrane preparation with different ion exchange groups and main chains.2)Following the above research,we further regulated the molecular structure of side chains by replacing single long alkyl chains into three short alkyl chains.We hope to this multi-alkyl side chain can increase the hydrophobicity of resulting membranes,leading to more distinct micro-phase separation and increase the steric hindrance surrounding functional groups to improve alkaline stability.Unexpectedly,desired result was not achieved for this structure.But it is concluded that for AEMs with alkyl side chains,changing single long alkyl chain into multi short alkyl chains can not improve membrane properties while may decrease its properties instead.This established foundation for designing membrane structure in the further investigation.3)Atom transfer radical polymerization is applied for the synthesis of hyper-branched oligomer HB-PVBC.Then the hyper-branched AEM HB-QPVBC was obtained by linking the hyper-branched oligomers HB-PVBC into spatial network using N,N,N',N",N"-pentamethyldiethylenetriamine(PMDETA)as the crosslinker.Compared to conventional linear AEMs,this hyper-ranched membrane exhibited higher hydroxide conductivity,lower water swelling and better alkaline stability.For example,its hydroxide conductivity is up to 84.7mS/cm,while the corresponding water uptake is as low as 35.2wt%.Furthermore,after immersed in lmol/L NaOH at 80 0C for 20 days,its ionic conductivity didn't show obvious change.The excellent performance profits from its specific hyper-branched structure.4)In the above investigations,we merely choose a common multi-amine crosslinker PMDETA to get the hyper-branched AEM HB-QPVBC.While in our initial trial,we found that the quaternization reagent is the key to membrane-forming performance.Theoretically,changing the structure of quaternization agent can regulate membrane properties.While recent investigation regarding side-chain type AEMs indicates that the introduction of alkyl side chains can increase the ionic conductivity and alkaline stability.We therefore prepared a serials of hyper-branched membranes using different diamines with variable carbon spacers(2C,6C,10C)to investigate the influence of quaternization agents on the res ting membranes.It is observed that with the increase of carbon spacer length,the resulting membrane showed worse mechanical property,higher water uptake,higher ionic conductivity and better alkaline stability.