Membrane/ionomer proton conductivity water uptake

The PEM are usually based on thin ionomer (polymer functionalised with acidic function) films. The main roles served by membrane are to separate the electrodes, to allow proton transportation from anode to cathode and to create a barrier against the passage of gases or fluids (e.g. methanol). Nevertheless the ionomer itself does not provide any appreciable conductivity and so must be swollen by molecules, e.g. water, to ensure proton conductivity. Water uptake by the membrane is therefore one of the most essential parameters in obtaining high conductivity levels. 

Cornelius et al. have synthesized a series of unique poly(phenylene)-based polyelectrolytes by Diels-Alder polymerization followed by post-sulfonation (Fig. 7.12) [29-32]. The ionomers are composed of sulfonated, highly phenylated poly(phenylene)s and do not carry any heteroatoms as their constituents except for the sulfonic acid groups. The complete aryl backbone resulted in a tough rigid-rod material with no Tg below the decomposition temperature. The stiffness of the ionomer backbone did not negatively affect the membrane properties such as water uptake (21-137%, in water) and proton conductivity (13-123 mS/cm, in water at 30°C) with lECs ranging from 0.98 to 2.2 meq/g. 

For instance, the Dow experimental membrane and the recently introduced Hyflon Ion E83 membrane by Solvay-Solexis are "short side chain" (SSC) fluoropolymers, which exhibit increased water uptake, significantly enhanced proton conductivity, and better stability at T > 100°C due to higher glass transition temperatures in comparison to Nafion. The membrane morphology and the basic mechanisms of proton transport are, however, similar for all PFSA ionomers mentioned. The base polymer of Nation, depicted schematically in Figure 6.3, consists of a copolymer of tetrafluoro-ethylene, forming the backbone, and randomly attached pendant side chains of perfluorinated vinyl ethers, terminated by sulfonic acid head groups. °... 

The built-in and operation stresses are the consequences of the large swelling and shrinkage of the ionomer membrane when it uptakes and loses water. This is frequently referred to as dimensional instability in the literature. Water in the PFSA membrane is an essential ingredient of its proton conduction behavior. Water affects the morphology13,14 of the ionic clusters (at nanoscale) which... 

To enhance membrane mechanical and hydro-thermal stability, Jiang et al. prepared a blend of side-chain sulfonated PFCB block copolymer and a PVDF fluoropolymer [129]. The chemical structure of the side-chain sulfonated PFCB ionomer is shown in Scheme 6.32. They evaluated the membrane s fiandamental properties, such as proton conductivity, gas permeability, water uptake, and... 

Guiver et al. of National Research Council, Canada developed comb-shaped poly(arylene ether) electrolytes containing 2-A sulfonic acid groups on aromatic side chains (d) [76]. Their membranes showed relatively high proton conductivity and well-developed and continuous ionic domains. However, trade-off relationship between water uptake and proton conductivity of their membranes was not better than that of Nafion. In order to pronounce the hydrophilic/hydropho-bic differences, another series of comb-shaped aromatic ionomers with highly fluorinated main chains and flexible poly(a-methyl styrene sulfonic acid) side chains were developed [77]. The membranes seemed to have better properties than their previous version, however, chemical instability of the side chains needed to be improved. 

The most commercially successful are membranes Nafion (Du Pont, USA). Nafion is perfluorosulfonic ionomer with high proton conductivity and chemical and mechanical stability [6]. However, limited operation temperature, low water uptake, and high cost are disadvantages of the perfluorosulfonic polymer. 

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