טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentAmel Alina
SubjectStudy of Anion Exchange Membranes for Fuel Cells
DepartmentDepartment of Materials Science and Engineering
Supervisor Professor Yair Ein-Eli
Full Thesis textFull thesis text - English Version


Abstract

Fuel cell technology has been recognized as a promising clean energy conversion technology in next-generation energy systems. Recently, there has been growing interest in anion exchange membranes for use in electrochemical systems such as alkaline anion exchange fuel cell devices. At present, there are a lot of obstacles to the use of anion exchange membranes including low chemical stability, low ionic conductivity and water management issues. In this research, we wish to investigate some of these central challenges, so it could provide important insight into the design principles of these new membranes. The presence of sulfone linkage, located in the backbone of the membrane was investigated. Two membranes were examined, quaternary ammonium poly(sulfone) and quaternary ammonium poly(phenylene oxide). Conductivity, thickness and swelling measurements were examined at various temperatures for both types of samples and revealed that the sulfone linkage cause an increase in these parameters, due to his polar nature. Chemical analysis indicated that poly(sulfone) membrane degraded much faster under high temperature and high pH conditions. Poly(sulfone) showed degradation at both the cationic groups and the ether linkage in the backbone structure after 150 h at 60 °C, while the poly(phenylene oxide) membrane showed degradation merely at the cationic groups after aging for 1000 h at 60 °C, under high pH conditions. The sulfone linkage, being an electron withdrawing group has a negative influence on stability of the ether group in thermal and alkaline environment. Another aspect concerning water management was studied. This issue was investigated by considering two different types of crosslinkers, one has an aminoether group and the other, has a trimethylhexadecyl amine group. These two crosslinkers have a fundamental difference, polarity and hydrophilicity. An aminoether crosslinker is more polar and therefore, will improve membrane's water uptake and conductivity. While water uptake and conductivity were found to be higher for the amino-ether crosslinker, degradation measurements indicated that the membrane that is crosslinked with an amino-ether group degraded faster at high temperature and pH conditions. Conductivity mechanism was investigated through quaternary ammonium poly(sulfone) based anion exchange membrane in Cl- and HCO3- forms. It was found that the membrane in both of the forms has a surface with highly connective island-like structure. In addition, the AEM in its HCO3- form showed higher water uptake values than in its Cl- form across the temperature range of 25-80 oC. Computational model was developed in order to understand the conductivity mechanism and the relevant parameters that limit ion transport in these materials. Together with the experimental results, it was found that only 40% of the ions are free for ionic conductivity, while 60% of the ions are bound to the cationic groups, therefore unavailable to participate in the conduction process. In addition, a limited water uptake decreases the conductivity by a factor of 5-8, and an additional factor of 2 is lost, due to the low connectivity of the hydrophilic channels.