Preparation And Performance Of Addition-Type Polynorbornene With Imidazolium Cation Cluster Anion Exchange Membranes

As an emerging energy conversion device,fuel cell has become the preferred clean energy technology in the 21st century due to its high efficiency and environmental protection.As the core component of fuel cell,anion exchange membranes（AEMs）are responsible for ion conduction and fuel barrier between cathode and anode,and their performance is of great significance to the entire fuel cell.At present,high-performance AEMs for fuel cells still face problems such as low conductivity,insufficient thermal alkali stability,poor dimensional stability.In addition,there is a trade-off relationship among them.In view of the above problems,in this paper,based on the concept of molecular structure design,an addition-type polynorbornene block copolymer and a long flexible side chain were designed.Combined with ion clusters with steric hindrance effect and electronic effect and cross-linked network structure,an imidazolium ion cluster polynorbornene anion exchange membrane was prepared.The main research work is as follows:（1）Brominated long alkyl side chain norbornene monomer（NB-O-Br）and long alkyl side chain norbornene monomer（NB-O-Hex）were synthesized by Williamson etherification reaction.The two norbornene monomers were polymerized under the late transition metal catalyst system（η³-allyl）Pd（PPh₃）Cl/Li[B（C₆F₅）₄]·2.5Et₂O（Li[FABA]）to obtain post-functionalized polynorbornene diblock copolymer aP（NB-O-Br-b-NB-O-Hex）.This strategy was employed to construct the s-triazine based triimidazolium cation cluster using the s-triazine ring.Furthermore,a series of s-triazine based triimidazolium cation cluster functionalized addition-type polynorbornene anion exchange membranes（aPNB-T-2MIm-x）with different IEC were prepared by ionic functionalization reaction and crosslinking reaction.TGA showed that the initial thermal decomposition temperature of the prepared AEMs was higher than 220 ℃,indicating good thermal stability.TEM images showed that AEMs with long side chains,densely grafted cationic clusters and block structures had obvious microphase separation morphology and more efficient ion transport channels.The ion conductivity of aPNB-T-2MIm-1.87 membrane with IEC of 1.87 mmol g^-1 reached 108.76 m S cm^-1at 80 ℃.At the same time,the crosslinking strategy also ensured the dimensional stability at high conductivity.The swelling ratio of aPNB-T-2MIm-1.87 membrane at80 ℃ was only 11.93%.In addition,the all-hydrocarbon skeleton of polynorbornene combined with the imidazolium cation with steric hindrance effect and electronic effect,which improved the alkali resistance stability of the membrane.After immersion in 1M NaOH solution at 80 ℃ for 576 h,the ion conductivity of the aPNB-T-2MIm-x membranes remained above 82.8%of the initial value.（2）Based on the above research results,the strategy combining grafting dense cationic clusters on long side chains with crosslinking by ionic reaction was an effective method to improve ionic conductivity,long-term alkali stability and dimensional stability.Therefore,this chapter used more dense cationic clusters to prepare AEMs.Based on the core structure of cyclotriphosphazene,hexaimidazolium cation clusters were designed and constructed.Combined with diblock polynorbornene copolymers containing long alkyl bromide side chains for post-functionalization,a series of cyclotriphosphazene-hexaimidazolium ion cluster functionalized addition-type polynorbornene anion exchange membranes（aPNB-CTPP-2MI-x）with different IEC were prepared by ion functionalization reaction and crosslinking.TGA showed that the initial thermal decomposition temperature of the prepared AEMs was higher than 240℃,indicating excellent thermal stability.TEM showed that a more obvious microphase separation structure was constructed in the membrane,which had better ion conductivity.The membrane aPNB-CTPP-2MI-1.93 with IEC of 1.93 mmol g^-1 had a conductivity of 115.09 m S cm^-1 at 80 ℃.In addition,aPNB-CTPP-2MI-x membranes can obtain higher ionic conductivity than aPNB-T-2MIm-x membranes under comparable IEC.This was due to the high concentration of imidazole groups in the hexaimidazole ion cluster,which was easy to form a larger and interconnected ion domain,build a more developed ion transport channel,and further improve the conductivity.The cross-linked network structure within the AEMs effectively inhibited excessive swelling under high IEC and improves dimensional stability.The swelling ratio of aPNB-CTPP-2MI-1.93 was only 12.83%at 80 ℃.The steric hindrance of cyclotriphosphazene structure and the skeleton of all-hydrocarbon polynorbornene without electron-withdrawing groups greatly enhanced the alkali resistance stability of AEMs.After soaking in 1 M NaOH solution at 80 ℃ for 576 h,the aPNB-CTPP-2MI-x membranes retained more than 85.7%of the ion conductivity,showing good alkali resistance stability.