Polymer Electrolyte Membranes Containing Benzimidazole Moiety for Fuel Cells

Abstract

This thesis focused on developing and studying polymer electrolyte membranes containing benzimidazole moiety for fuel cells Benzimidazole is amphoteric and exists two equivalent tautomeric forms which can be acted as either proton exchange site or anion exchange site depending on the transfer of a proton (deprotonation and protonation) Three novel polybenzimidazole (PBI) based polymer electrolyte membranes have been developed including phosphoric acid doped MWNT/PBI composite membranes for high temperature proton exchange membrane fuel cell (HT-PEMFC) phosphoric acid doped asymmetric PBI membranes for HT-PEMFC and quaternized PBI membranes with imidazolium and benzimidazolium functional groups for anion exchange membrane fuel cell (AEMFC) HT-PEMFC and AEMFC are the two promising variants of the polymer electrolyte membrane fuel cell which have possibilities to meet the challenges associated with electrode reaction kinetics use of expensive Pt catalyst and water management Firstly composite membranes used for proton exchange membrane fuel cells comprising of PBI and carbon nanotubes with certain functional groups were studied They could enhance both the mechanical property and fuel cell performance at the same time Two kinds of composite membranes including sodium poly(4-styrene sulfonate) functionalized multiwalled carbon nanotubes (MWNT-poly(NaSS))/ PBI and imidazole functionalized multiwalled carbon nanotubes (MWNT-imidazole)/PBI composite membranes were prepared The functionalization of carbon nanotubes involving non-covalent modification and covalent modification were confirmed by FITR XPS Raman spectroscopy and TGA Compared to unmodified MWNTs and MWNT-poly(NaSS) MWNT-imidazole provided more significant mechanical reinforcement due to its better compatibility with PBI For MWNT-poly(NaSS)/PBI and MWNT-imidazole/PBI composite membranes at their saturated doping levels the proton conductivities were up to 5 1 × 10-2 and 4 3 × 10-2 S/cm at 160°C under anhydrous condition respectively which were slightly higher than pristine PBI (2 8 × 10-2 S/cm) Also MWNT-poly(NaSS)/PBI and MWNT-imidazole/PBI composite membranes showed relatively improved fuel cell performance at 170 °C compared to pristine PBI Secondly a novel asymmetric PBI membrane used for HT-PEMFC has been successfully fabricated by a soft-template method using ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][TFSI]) as porogen The asymmetric PBI membrane typically exhibits a double layer structure comprising a dense layer and a porous layer with a distinguishable boundary The morphology and asymmetry of the porous structure has been characterized by SEM micrographs The density difference between the polymer matrix and the porogen can be considered as the driving force for developing the asymmetrical structure The phosphoric acid-doped asymmetric PBI with a high porosity exhibited considerably enhanced doping level and proton conductivity For example a doping level of up to 23 6 and a proton conductivity as high as 6 26 × 10-2 S/cm were achieved Moreover the crosslinking modification of asymmetric PBIs had facilitating effects on mechanical strength and oxidative stability which were investigated We have also demonstrated fuel cell performance of membrane electrode assembly (MEA) based on the asymmetric PBI at elevated temperatures under anhydrous conditions in the present work Thirdly a new series of quaternized PBIs having imidazolium moieties in the main-chain and/or in the side group were synthesized for use as anion exchange membrane (AEM) for fuel cells The polymer structures were characterized by 1H NMR FTIR and EDX analyses The degree of imidazolium functionalization (DIF) of the quaternized PBI was also determined The properties required for AEM such as ion exchange capacity (IEC) water uptake swelling ratio hydrated number and ionic conductivity were measured The IECs of the quaternized PBIs were in the range of 0 96~1 49 mmol/g The highest ionic conductivity of 2 72 × 10-2 S/cm was achieved at 80?C Besides the thermal stability mechanical properties and alkaline stability of the quaternized PBI membranes were investigated The results revealed that the thermal stability and mechanical properties of the membranes were acceptable but the alkaline stability of the (benz)imidazolium moieties needs to be improved

Date of Award	2014 Oct 27
Original language	English
Supervisor	Lien-Chung Hsu (Supervisor)