Novel Side-Chain-Type Hydroxide Exchange Membrane Materials For Direct Methanol Fuel Cells: Syntheses And Properties Studies

Fuel cell has been regarded as an effective approach to resolve the recentenergycrisis and environment problems, because of its high energy density, highconversion efficiency, and low pollution levels. Among the several kinds of fuel cells,polymer electrolyte membrane fuel cells have attracted the most attention. Comparedwith proton exchange membrane fuel cells (PEMFCs), hydroxide exchangemembrane fuel cells (HEMFCs) show some unique characteristics. The mostsignificant advantage is the inherently faster kinetics of the oxygen reduction reaction,which allows using non-noble and low-cost metal electrocatalysts in the HEMFCs.My research is mainly focusing on hydroxide exchange membranes for fuel cellapplications.Locating the electrolyte groups on the side chain and cross-linking are twoimportant strategies to prepare polymer electrolyte membranes with enhancedperformances. Side-chain-type structures can help to enhance the separation of thehydrophilic regions and the hydrophobic polymer backbone, which results inimproved conductivity and stabilities. Cross-linking can help to suppress theexcessive dimensional changes and fuel permeability of the membranes. In Chapter Three, We synthesized a new type of polyelectrolyte of which thequaternary ammonium groups are on the side chain, to decrease the steric hindranceof aromatic backbones and promote the separation of hydrophilic and hydrophobicphases, and finally to improve the hydroxide conductivity at a low level of IEC. Aseries of poly(ether ether ketone) anion exchange membranes with tunable IEC(Ion-exchange capacity) values were polymerized from a novel bisphenol monomer(4-methyl)phenylhydroquinone. The resulting PEEK-Q-xx membranes show gooddimensional stability. The highest swelling ratio (PEEK-Q, with an IEC value of0.90mequiv. g-1) is only9.0%at80oC, at which temperature, the hydroxide conductivityof PEEK-Q membrane is0.031S cm-1. The methanol permeability values of themembranes are all below1.5×10-7cm2s-1, which are much lower than that of Nafion117. These properties make the membranes good candidate materials for anionexchange membranes for alkaline direct methanol fuel cells.In Chapter Four, we synthesized a novel series of HEMs combined thecross-linked structure and aromatic-side-chain. A series of novel poly(ether etherketone) copolymers containing methyl groups on the sidechain were preparedbased on a new monomer (3,4-dimethyl)phenylhydroquinone. Then a series ofhydroxide exchange membranes with different IEC values were obtained throughbromination and quaternary amination of the copolymers.After that, for thepurpose of enhancing the dimensional stability and methanol resistance of themembrane, we prepared cross-linked membranes through the Friedel–Craftsreaction between bromomethyl groups and aromatic rings. The properties of the membranes related to fuel cell application were evaluated in detail. All themembrane showed good conductivities.Moreover, the cross-linked membranesexhibit better dimensional stabilities and selectivities. Among those membranes,xPEEK-diQ-100showeda high conductivity (0.036S cm-1at80oC), low swellingratio of6.6%and a methanol permeation coefficient of2.9×10-7cm2s-1. Theoutstanding properties indicated that the application of PEEK-diQ-xx membranesin fuel cells was promising.