Syntheses And Properties Of Side-chain-type Anion Exchange Membranes Functionalized With Guanidinium And Pyrazolium Cations

In recent years, with the advantages of using non-noble metal electrode catalyst and low fuel leakages, anion exchange membrane fuel cell (AEMFC) has caught more and more attention, and is expected to replace the present well-developed proton exchange membrane fuel cell. However, as one of the key components in AEMFC, anion exchange membrane (AEM) often suffers from low ionic conductivity and poor long-term durability, which limits the further development of AEMFC. Therefore, to explore the relationship between the structure and performance of AEM, and exploit AEM with excellent comprehensive performance is one of the main research directions in the fields of AEMFC. In this dissertation, we investigated the influence of spacer length and properties between functional cations and polymer backbone to the corresponding AEM properties. On the other hand, we evaluated the chemical stability of novel azoliums, then selected the most chemical stable pyrazolium as functional cations tethering to polymer backbone to the formation of AEM, and further evaluated the corresponding AEM's alkali durability. Thus, the following works have been carried out:1) We synthesized a series of side-chain-type anion exchange membranes GPES-nC(n=1,3,9) and GPES-EO via the "click chemistry", with the guanidinium as the functional cation and polyether sulfone as the polymer backbone, mainly to investigate the influence of the spacer type and length to anion exchange membrane performance. The results illustrated that when the spacer was alkyl chain, AEMs showed low water uptake, and the water uptake increases as alkyl chain length increasing. On the other hand, GPES-EO with the hydrophilic ethylene oxide side chain showed a relatively higher water uptake than GPES-nCs. In terms of ionic conductivity, GPES-3C membrane showed higher conductivity than the GPES-1C and GPES-9C membrane, with the conductivity of 18.9 mS·cm-1 at 30 ?, and the GPES-EO membrane exhibited the highest conductivity of 41.4 mS·cm-1 at 30 ?.AFM measurements proved that the membrane GPES-3C and GPES-EO exhibited well hydrophilic-hydrophobic phase separation structure, which accordance well with the conductivity results.2) We evaluated the alkaline stability of thiazolium, imidazolium and pyrazolium salts by in situ 1H NMR, and the results showed that alkyl pyrazolium salt exhibited outstanding alkaline stability. For example, 1,3,5-trimethyl-2-ethyl pyrazolium bromine salt showed no distinct decomposition in 5 M NaOH/D2O solution at 80 ?for 10 d. So we further selected pyrazolium cation as functional group, connecting it to the PPO backbone, to prepare for anion exchange membrane PPO-Py. This membrane exhibited low water uptake and good dimensional stability, and the choride ionic conductivity was 5.0-23.0 mS·cm-1 at 30-80 ?. AFM results showed that the membrane formed well phase separation structure. Unfortunately, in the subsequent evaluation of the alkaline stability of the PPO-Py, we found that the ionic conductivity decreased sharply to 18% after 3 d in 1 M NaOH solution at 80 ?. Meanwhile, XPS confirmed that the decline of conductivity was caused by decomposition of pyrazolium salt in alkaline condition.