Synthesis And Properties Of Anion Exchange Membranes With Exceptional Alkaline Stability

Anion exchange membrane fuel cells are more attractive than proton exchange membrane fuel cells because nonprecious metals can be used in place of precious metals as catalysts.However,as the core component of anion exchange membrane fuel cells,anion exchange membranes still face many problems.The two most important ones are:the low conductivity and the poor alkaline stability.This instability of membranes is believed to be primarily caused by two factors:degradation of the polymer backbone and degradation of the functional cationic groups.From the perspective of improving the alkaline stability of anion exchange membrane,the influence law of polymer backbone and cationic group structure on the comprehensive performance of anion exchange membrane was deeply studied in this paper,and some innovative research results were obtained.The specific contents are as follows:1.Anion exchange membranes with quaternary ammonium group(QA)and two kinds of bulky imidazolium(MIm,PIm)were designed and prepared.It has been discovered that PPO-PIm-0.4 film-forming ability is poor because of the large volume of PIm.Due to the ylide and Huffman degradation of the QA,the hydroxide conductivity of PPO-QA-0.4 decreased to 35%of the initial value after immersion in 2 mol/L NaOH solution at 80? for 30 days.Due to the crosslinking of polymer chains and the degradation of cationic groups,the hydroxide conductivity of PPO-MIm-0.4 was decreased to 10%of the initial value after immersion in 2 mol/L NaOH solution at 80? for 30 days.The the peak power densities of membrane electrode assemblies of PPO-QA-0.4 and PPO-MIm-0.4 with 0.5 mg/cm2 Pt loading was 351 mW/cm2 and 17 mW/cm2 respectively at 60? with H2/O2 flow rate of 800/800 sccm under 100%RH with no back pressure.2.With aryl ether-free poly(fluorene alkylene)backbone,four kinds of anion exchange membranes PFBA-QA-0.4,PFBA-Py-0.4,PFBA-Pi-0.4,and PFBA-MIm-0.4,two kinds of binders PFBA-QA-0.7 and PFBA-MIm-0.7,and four kinds of cationic polymers PFA-QA,PFA-Py,PFA-Pi and PFA-MIm for alkaline stability test were designed and prepared.The computational radius of QA,Py,Pi and MIm were 2.116 A,2.776 A,2.982 A and 7.087 A,respectively.Because of the large size of the cation groups,the chain entanglement density of AEM with bulky imidazolium(PFBA-MIm-0.4)is lower than other three membranes,while the water uptake of PFBA-MIm-0.4 is much higher than other three membranes.All four membranes,including anion exchange membrane with quaternary ammonium group,were stable for at least 30 days in a solution of 5 mol/L NaOH at 80?.The imidazolium cationic polymer exhibited better alkaline stability than the quaternary ammonium cationic polymer in homogeneous systems sealed in nuclear magnetic tubes.The peak power densities of membrane electrode assemblies of PFBA-QA-0.4 and PFBA-MIm-0.4 with 0.5 mg/cm2 Pt loading was 610 mW/cm2 and 32 mW/cm2 respectively at 60? with H2/O2 flow rate of 800/800 sccm under 100%RH with no back pressure.3.PFBA-QA-x,PFBA-MIm-x and PFBA-PIm-x with different IEC were prepared.Because of higher molecular weight and better molecular chain flexibility,the PFBA-PIm-x exhibits better film-forming ability than PPO-PIm-0.4.For the three series of PFBA membranes,the higher the IEC,the higher the water uptake and swelling ratio,and the greater change with the increase of temperature.With the same IEC,the water uptake and swelling ratio of PFB A membranes were lower than that of PPO membranes.The hydroxide conductivity follows the order of?(PFBA-QA-x)>?(PFBA-MIm-x)>?(PFBA-PIm-x)and ?(PPO-QA-x)>?(PPO-MIm-x)at the same hydration number(?),due to the alkalinity of QA>MIm>PIm.At the same ? value,the conductivity of PFBA membranes is higher than that of PPO membranes.4.By connecting several quaternary ammonium groups on each side chain,bi-quaternary ammonium anion exchange membrane(PFBA-bQA-0.2)and tri-quaternary ammonium anion exchange membrane(PFBA-tQA-0.12)with large ion concentration area and micro-phase separation structure were successfully prepared.The hydroxide conductivity at 30? of PFBA-bQA-0.2 and PFBA-tQA-0.12 are 63 mS/cm and 62 mS/cm respectively,which are lower than 77 mS/cm of PFBA-QA-0.4.The ion concentration strategy did not improve the conductivity of anion exchange membrane...