Molecular Design Of Alkali-resistant Cation And Its Application In Anion Exchange Membranes

Anion exchange membrane (AEM) is an important component of alkaline anion exchange membrane fuel cell,which plays a key role in isolating fuel and oxidant and conducting hydroxide.The ideal AEM needs to have excellent alkaline stability,high ionic conductivity,suitable water absorption,good dimensional stability and thermal stability.Among them,excellent alkaline stability is the key to ensuring long-term stable operation of fuel cells.At present,the usage of anion exchange membranes still faces the problem of poor alkaline stability,which severely restricts its application in fuel cells.The alkaline stability of anion exchange membranes mainly depends on two factors:the stability of ionic groups and polymer backbone.In this paper,from the perspective of improving the alkaline stability of AEMs,the relationship between the structure of ionic groups and alkaline stability is systematically studied by the molecular structure design of organic cationic groups,and a variety of AEMs based on alkali-resistant ionic groups have been developed via polymer modification and polymerization of functional monomers.The main research contents and results are as follows:1.Thirteen guanidinium model compounds (1a-1d,2a-2e and 3a-d) with diffeerent substituents at N1/N2/N3 positions were designed and prepared.Through 1H NMR and mass spectroscopy,the degradation process of the model compounds was studied in 1 mol L-1 alkaline solution at 60?,and the relative stability sequence of the model compounds was summarized.It showed that the ring structure of cyclobutyl weakens the freedom of movement of the substituents,reduces the steric hindrance effect,and is not conducive to the resistance of the guanidinium to the attack of hydroxide;the guanidiniums substituted by the ethyl group at the N1/N3 position behave outstanding alkaline stability and much better than methyl or cyclobutyl substituted ones;the influence of N2 substituent follows the following rules:isopropyl> ethyl> methyl> benzyl> phenyl.Studies on the degradation mechanism of guanidinium salts show that the degradation follows a nucleophilic addition-elimination process:the center carbon of guanidinium is attacked by hydroxide to form a tetrahedral intermediate,and then the protonated amine is eliminated,and the remaining part forms urea.2.Based on the study of guanidinium cations,two guanidiniums with excellent stability (2c and 2e) were selected and introduced into the polymer backbone by the following method.First,starting from the commercial polysulfone,the boronated polysulfone PSU-Bpin was prepared by the iridium-catalyzed boronization reaction.Then two aryl iodide monomers containing guanidinium groups were subjected to Suzuki-Miyaura coupling reaction with PSU-Bpin to prepare two side chain type anion exchange membranes PSU-P-EG and PSU-P-PG.The results show that the side chain structure endows PSU-P-EG and PSU-P-PG very evident phase separation structure,which show higher conductivity than the main-chain type anion exchange membrane.In addition,PSU-P-PG can be kept in a 1 mol L-1 alkaline solution for 30 days without significant degradation,and the conductivity loses of PSU-P-EG is less than 4.5%,showing their excellent alkaline stability.3.In order to obtain guanidinium-based AEMs that simultaneously own high ion exchange capacity (IEC),high conductivity and high dimensional stability,two bi-guanidinium cross-linkers with different chain lengths are designed.Brominated polyphenylene oxide (BPPO) is prepared by bromomethylation of polyphenylene oxide,and then nucleophilic substitution reaction were carried out between the bi-guanidine cross-linkers and the bromomethyl groups in BPPO to construct the polymer network structure.Four bi-guanidinium based cross-linked membranes were prepared by adjusting the chain length of biguanide cross-linker and the degree of bromomethylation of BPPO.The research results show that the four cross-linked AEMs have good dimensional stability,and the maximum swelling ratio at 80? is less than 15%.Stability studies show that only a small amount of degradation occurred in the initial stage during alkaline treatment,indicating four cross-linked AEMs have good alkaline stability.4.A guanidinium functionalized monomer by fluoroanilation-quaternization reactions was designed and prepared.Using this monomer,AEMs GPP-co-PAEK x with microblock structure and ionic groups located in the polyphenylene chain segment were successfully prepared by Ni (0) catalyzed coupling method.The results show that the introduction of long hydrophobic non-ionic chain segments into polymer main chains endows GPP-co-PAEK x membranes microblock structure and good phase separation structure,which is conductive to restrict the excessive swelling and achieve good dimensional stability.5.A novel imidazolium ionized monomer PhIM [PF6] substituted with propylbenzene at the N1/N3 position was designed and prepared.Under the condition of superacid catalysis,PhIM [PF6] was copolymerized with 1,1,1-trifluoroacetone and the polyarylimidazolium Poly-PhIM [OH] was obtained.Block copolymer Poly-PhIM-b-biPh [OH] and random copolymer Poly-PhIM-co-biPh [OH] can be obtained by introducing biphenyl units as a comonomer and adjusting the feeding sequence during the polymerization process.The results reveal the relationship between microstructure and water uptake,swelling ratio and ion conductivity.The polyarylimidazolium Poly-PhIM [OH] has excellent alkali-resistance.Even after 100 days of treatment in a strong alkaline solution at a high temperature (80?,10 M NaOH aq.),no evident chemical structure changes were found.