Proton Exchange Membranes With High Methanol Resistance Property: Preparation And Performance Studies

Proton exchange membrane fuel cells are the most attractive power source because of the high energy efficiency and environmentally friendly energy. The core material of proton exchange membrane fuel cells is proton exchange membrane, which acts as a medium to transport proton and an obstacle to prevent the transporting of electron, fuel and oxidant. The commercialy available proton exchange membranes such as Nafion developed by DuPont have been the most studied electrolyte membrane because of their high proton conductivity and good chemical stability. However, the expensive cost and the poor methanol resistance make them impossible for industrial application. Sulfonated aromatic polymers have been the most studied materials because of the high proton conductivity, good mechanical property and low cost. Unfortunately, the prevalent problem of them is that the high sulfonated polymers have poor methanol resistance. Many methods were taken to overcome this problem, such as to prepare new polymer, cross-linked membranes and acid-base composite membranes.Firstly, we have synthesized a novel bisphenol monomer containing carboxyl group and prepared a siries of sulfonated poly(ether ether ketone) containing carboxyl group. The introduction of carboxyl group makes a strong interaction between polymer chains, which brings many excellent properties of polymer membranes. C-SPEEK-4 which has the highest sulfonated degree, shows lower water uptake and good dimensional stability. The proton conductivity of C-SPEEK-4 is higher than that of Nafion117, however, the methanol permeability of C-SPEEK-4 is merely 20%of Nafion117.The copolymerization method of preparing sulfonated poly(ether ether ketone) containing carboxyl group needs long reaction time and has low probality. so we have prepared poly(ether ether ketone) containing carboxyl group through aromatic nucleophilic substitution reaction, and prepared sulfonated poly(ether ether ketone) containing carboxyl group by post-sulfonation process. The poly(vinyl alcohol) was choosed as a cross-linker. It is observed that with increasing the sulfonation time, the membranes displayed improved proton conductivity and water uptake. Meanwhile, the mechanical property of membranes is reduced. For cross-linked membranes, with increasing the content of cross-linker, the cross-linked membranes displayed improved mechanical property. The proton conductivity of cross-linked membranes is slightly lower than that of pristine sulfonated membrane. However, the methanol permeability of cross-linked membranes is greatly lower than that of pristine sulfonated membrane, the methanol permeability of C-SPEEK-8 with the highest cross-linked degree is five times lower than that of pristine sulfonated membrane.We have prepared sulfonated poly(ether ether ketone) (IEC=1.71 mequiv. g-1 through post-sulfonation process. NaBH4 was choosed as a reducing agent to prepare sulfonated poly(ether ether ketone) with hydroxyl groups. The thermal cross-linked method was taken to prepared self-cross-linked membranes under acid environment. This method needn't any cross-linkers or additional catalysts. All cross-linked membranes have maintained IEC. reduced water uptake, improved dimensional and methanical property. The cross-linked membranes exhibited slightly decreased proton conductivity due to the lower water uptake. However, the cross-linked membranes displayed greatly reduced methanol permeability. Meanwhile, the selectivity result indicated that the selectivity of all cross-linked membranes is higher than that of pristine membrane.We have prepared sulfonated poly(ether ether ketone) and poly(ether ether ketone) through aromatic nucleophilic substitution reaction. Bromomethylated poly(ether ether ketone) (PEEK-Br) was synthesized with a brominated agent of NBS and used as a macromolecular cross-linker. The cross-linked reaction was performed at 190℃without any catalysts. All cross-linked membranes with high cross-linked degree displayed greatly improved dimensional stability and mechanical property. In spite of the slightly reduced proton conductivity, all cross-linked membranes displayed greatly improved methanol resistance. The relative selectivity of all cross-linked membranes is higher than 1, SPEEK-4 membrane had the highest relative selectivity of 1.69.We have prepared benzimidazole grafted poly(ether ether ketone) (PEEK-BI) by the reaction between bromomethylated poly(ether ether ketone) (PEEK-Br) and 2-benzimidazolethiol. Acid-base composite membranes of SPEEK and PEEK-BI were prepared. The IEC of composite membranes decreased with increasing the content of PEEK-BI. All composite membranes displayed greatly reduced water uptake and swelling ratio, improved mechanical and oxidative stability. The relative selectivity of all cross-linked membranes is higher than 1. SPEEK-10 had the highest relative selectivity indicating that the best doping ratio is 10%.