Proton-Conductive Block Copolyphenylchinoxalines Ionomers for Fuel Cells and Electrolysis

Abstract

The proton exchange membrane fuel cell (PEMFC) is considered to be the most promising power source for portable and automotive applications. Currently, perfluorinated polymers such as Nafion are the state of the art materials. However, high operation temperatures, cause weakening of their properties. In addition, their shortcomings such as high cost and high methanol permeation property, limit their application. To remedy these problems, aromatic hydrocarbon polymers have been extensively studied as alternative materials for proton exchange membranes (PEMs). Recently, introduction of block copolymer structures to PEMs has been considered as a strategy to overcome the weak points of a random copolymer system. Block copolymers composed of hydrophilic and hydrophobic segments are expected to form ion transport channels due to hydrophilic/hydrophobic phase separated structures and show improved proton conductivity. Poly(phenlyquinoxalines) are a family of aromatic condensation polymers known their outstanding thermal and chemical stability. Based on this polymer class new sulfonated block copolyphenylchinoxalines with hydrophilic and hydrophobic segments have been developed. The block copolymers were synthesized via polycondensation reactions between 3,3`-4,4`tetraaminodiphenylether and two different tetracetones (tetrafluorphenylbis-phenylethandion and phenylbisphenylethandion. High molecular Blockcopolyphenylchinoxalines with 20 000 in number average molecular weight were obtained. The sulfonation of these blockcopolymers was carried out in a mixture of concentrated sulfuric acid and oleum (4:1). In dependence of the reaction time blockcopolyphenyl-chinoxalines with ion exchange capacities of 0,6 - 2,35 mmol/g could be prepared. All sulfonated blockcopoly-phenylchinoxalines show good solubility in dimethylacetamide (DMAC). For the preparation of the membranes a phase inversion process via immersion precipitation was developed. The membranes achieved proton conductivities comparable to Nafion. Additionally the water uptake, oxidation and dimensional stability were investigated and presented.