Preparation And Characterization Of Crosslinked Anion Exchange Membranes

Anion exchange membranes(AEMs)are the core modules of the alkaline fuel cells,and determine the working life and power density of fuel cells.However,there are still many drawbacks in the AEMs,such as poor alkaline stability and insufficient dimensional stability,especially low ion conductivity.A direct method for improving ion conductivity is to increase ion exchange capacity(IEC).However,a high IEC will result in high water uptake and great swelling in water surroundings,which is unfavorable to the dimensional stability and alkaline stability of AEMs.That is,there is a trade-off relationship between ion conductivity and dimensional stability as well as alkaline stability.Therefore,an approach is strongly required to prepare high-performance AEMs that break the trade-off relationship of ion conductivity with alkaline stability and dimensional stability.Crosslinking is one of effecitive methods to adjust the membrane structure,and has been widely used in the preparation of AEMs recently.Usually,crosslinking can greatly restrain the water uptake and swelling ratio,and thus improve mechanical properties and alkaline stability of AEMs.,Besides,structure and performances of AEMs can be controlled by crosslinkers and crosslinking degree.In this thesis,a series of crosslinked AEMs was prepared and charectrized well.Structure-performance relation of AEMs was studied in detail.The main works and results are showed as follows.(1)A novel random microporous DPM/DMBP-TB copolymer with rigid structutre was designed and synthesized by using 4,4'-diaminodiphenylmethane(DPM)and 4,4'-diamine-3,3'-dimethyl-biphenyl(DMBP).This copolymer has good solubility in N-methylpyrrolidone(NMP)and abundant reaction sites for crosslinking and grafting.Then,a series of AEMs was prepared by crosslinking between DPM/DMBP-TB and multi-cation crosslinker.After that,physicochemical properties and electrochemical performances of the prepared AEMs was investigated in details.The results show that all the prepared AEMs have good dimensional stability,high chemical stability and satisfactory ion conductivity.Among them,the DPM/DMBP-QTB-1.5 AEM has the highest hydroxide conductivity(103.9 mS cm-1,80 ?)with low swelling ratio(20.8%,60 ?).Moreover,the DPM/DMBP-QTB-1.5 AEM remains over 80%hydroxide conductivity after immersion in alkaline solution for 1 month,showing its good alkaline resistance.(2)Effect of the length of hydrophobic crosslinker on the structure and performance of crosslinked AEMs was studied.Hydrophilic poly(phenylene oxide)(HPPO)was used as the matrix.Alkyl diamines were used as the hydrophobic crosslinkers.After that,physicochemical properties and electrochemical performances of the AEMs were investigated in details.The hydroxide conductivity of prepared HPPO-Cn AEMs increases first and then decreases with increasing length of crosslinker.The HPPO-C8 AEM crosslinked by 1,8-octanediamine has the highest hydroxide conductivity(145.7 mS cm-1,80 ?).All the AEMs have well mechanical properties and anti-swelling ability due to introduction of hydrophobic crosslinkers.To obtain higher ionic conductivity,hydrophobic PPO was introduced into the crosslinking network to construct a well-defined ion channels in the AEMs.As expected,the resulting HPPO/PPO-C8 AEM has a more distinguished micro-phase separation structure and bigger ion clusters than the HPPO-C8 AEM.Besides,the HPPO/PPO-C8 AEM has higher hydroxide conductivity of 157.2 mS cm-1 with low swelling ratio of 12.9%at 80 ?.Membrane-electrode assembly prepared by the HPPO/PPO-C8 AEM has the high power density of 222 mW cm-2 at 60 ? in the H2/O2 fuel cell.In conclusion,a series of crosslinked AEMs with high ion conductivity and low swelling were developed to break the trade-off relation between ion conductivity and dimensional stability of AEMs.Among them,the HPPO/PPO-C8 AEM,modified by both hydrophobic crosslinker and hydrophobic PPO chain,has the highest hydroxide conductivity of 157.2 mS cm-1 with low swelling ratio of 12.9%at 80 ?,showing a great potential for application in fuel cells.