Preparation And Properties Of Non-covalently Crosslinked Anion Exchange Membranes Based On Hydrogen-bonding Urea Groups

Nowadays,with the increasing attention of environmental protection and sustainable development,more and more attentions have been paid to the development of clean energy.As a new type of polymer electrolyte fuel cell,anion exchange membrane fuel cells?AEMFCs?have attracted extensive interest owing to the merits of fast oxygen reduction reaction?ORR?kinetics and the potential to utilize non-nobel metal electrocatalysts.As one of the key components in AEMFC,anion exchange membrane?AEM?often suffers from comparatively low ion conductivity,alkaline instability as well as poor dimensional stability.Therefore,it is of importance to banance the performance of ion contents,ion conductivity,and swelling resistance to achieve the high performance of AEM.In this dissertation,the urea group was introduced into the polymer side chain to construct the non-covalently hydrogen-bonding crosslinking in the AEM matrix,and the influence of hydrogen bond interaction of urea bond on the performance of AEM were systematically investigated.Specifically,the following works have been performed:1)The influence of the position of urea group in the side chain of polyphenylene oxide?PPO?on the performance of AEM was investigated.Benzyl urea-linkaged terminal quaternary ammonium?QA?-containing AEMs?b U-AEMs?and benzyl QA-linkaged urea containing AEMs?s U-AEMs?were synthesized and comparatively studied.The analyses by solubility test and variable-temperature infrared spectroscopy showed that urea-derived hydrogen-bonding interaction were formed in the AEMs.The hydrogen-bonding strength in b U-AEMs was higher than that of s U-AEMs,thus suppressed water uptake and improved dimensional stability were observed.For example,the chloride conductivity of b U-AEM-0.45 reached 50.3 m S cm^-1 at 90^oC,while its water absorption remained at a moderate value of 80.6 wt.%.In addition,similar to the traditional covalent crosslinking,the non-covalent interaction also significantly improve the the alkaline stability and thermal stability of the AEMs.2)The urea group was placed between the two quaternary ammoniums on the side chain of PPO backbone,aiming for the formation of non-covalent crosslinking network in the hydrophilic phase?cluster?of the AEM.This type of side chain interaction was expected to facilitate the aggregation of ion groups,which can be beneficial for the ion conducting in the membrane.The results showed that the ionic conductivity of bet U-AEMs was substantially improved,and the chloride conductivity of bet U-AEM-0.30 at 80^oC reached as high as 53.1 m S cm^-1,which is 4-fold higher than that of s U-AEM-0.30.Moreover,the IEC-normalized conductivity of bet U-AEMs is still more than 2 times higher than that of s U-AEMs,demonstrating the high ion-conducting efficiency.It should be noted that,although the water uptake and swelling of bet U-AEM were still at a high level,such as the water absorption of bet U-AEM-0.30 reaches 230.2 wt.%at 80^oC,there is no excessive swollen like s U-AEM was observed.There results demonstrated that the uera hydrogen-bonding side chain crosslinking is an efficient way to suppress the water absorption of the AEM,moreover,the ion-conducting efficiency were improved also.3)Blended AEMs were prepared by solution blending of triple quaternary ammonium containing AEMs?PPO-3QA?and urea-containing AEMs?b U-AEMs?.The results showed that the blended AEM exhibited high ionic conductivity,suppressed water absorption and low in-plane swelling ratio.The chloride conductivity of3QA-PPO-0.36/b U-PPO-0.30 was 57.7 m S cm^-1 at 80^oC,while the water uptake and in-plane swelling ratio were 111.8 wt.%and 8.6%,respectively.Unexpectedly,the through-plane swelling ratio of the 3QA-PPO-0.36/b U-PPO-0.30 was as high as 92.6%,exhibiting an distinct anisotropy.It was found by SEM that the microstructure of the AEM was orderly arranged in the length direction,but layered stacked in the thickness direction,which resulted in the cohesion in the length direction is much higher than that in the thickness direction.