Membranes, Aromatic Ionomer, Properties conductivity

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 group of GKSS Research Center Germany extensively researched the effect of a variety of inorganic nanoparticles on the properties of aromatic ionomer membranes [86-93]. Composite membranes were prepared from silicates and sulfonated poly(ether ketone)s or sulfonated poly(ether ether ketone)s. For DMFC applications, the composite membranes showed promising properties with lower methanol and water permeability and comparable (or higher) proton conductivity compared to the parent polymer membranes. The flux of water and methanol decreased with the increase in content of silicates. 

Many research studies have focused on improving the nanophase-separated structures between the hydrophilic and hydrophobic units to increase the proton conductivity of the aromatic ionomers under low RH. In this section, recent approaches to improve the membrane properties, especially the proton conductivity, which is usually the first characteristic considered when evaluating membranes for fuel cells, and morphology will be discussed as follows (1) multiblock SPES copolymers, (2) locally and densely SPES, (3) SPES with high lEC values and high free volume, (4) SPES with pendant perfluoroalkyl sulfonic acids, (5) cross-linked SPES, and (6) thermally annealed SPESs. 

Despite its higher lEC values, the proton conductivity of the membranes was lower than that of the Nafion membrane. The authors concluded that acidity was a crucial factor to determine the proton conducting properties. However, other structural factors, such as the hydrophobicity and flexibility of the main chain, would have to be optimized for further improving the proton conductivity of the aromatic ionomer membranes [51]. On the other hand, Ueda and coworkers reported a novel PES containing binaphthyl units with pendant perfluoroalkyl sulfonic acids (BNSH-PSA) for PEM [52]. The BNSH-PSA (1EC=1.91 meq./g) was prepared by the aromatic nucleophilic substitution reaction of l,T-binaphthyl-4,4 -diol and 4,4 -dichlorodiphe-nylsulfone, followed by bromination with bromine, and then the Ullmann coupling reaction with PSA-K (Scheme 4.19). 

This chapter is a review focussed on the development of ionomers based on aromatic polysulfones for their application as Polymer Electrolyte Membrane (PEM) in Proton Exchange Membrane Fuel Cells (PEMFC) or in Direct Methanol Fuel Cells (DMFC). Different types of synthesis routes have been discussed in this chapter in order to obtain ionomers based on polysulfones with variation in structural designs. Special attention is given to the impact of the structural design of the ionomer on various properties such as membrane morphology, thermo-mechanical stability and protonic conductivity of the membranes for their utilization as PEMs. 

Proton-conducting ionomers for polymer electrolyte membrane fuel cell (PEMFC) is one area in which extensive research is ongoing to modify aromatic main chain polymers to tailor their properties as proton conductors [1-... 

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