Polymer membranes sulfonation methods

The greatest interest in Nafion (Nf), a perfluorinated polymer with sulfonate groups, derives from its consideration as a proton conducting membrane in fuel cells to many sensor applications [37], The use of Nf and other polyanions to control ion-transport during electrode reaction is a recurring theme [38], The electrodeposition of platinum nanoparticles at the Nafion (Nf) modified glassy carbon (GC) electrode (GC/Nf/Ptnano) by the two-step method... 

From the overview presented above it can be found that there are numerous methods of preparation of polymeric photocatalytic membranes. However, it should be stressed that in case of polymer membranes there is always a danger of destruction of the membrane structure by UV light or hydroxyl radicals. This risk is associated with the reactor configuration. Application of a photocatalytic membrane requires irradiation of the membrane itself in order to perform the photodecomposition of pollutants. The lowest UV resistance is exhibited by membranes prepared from polyether-sulfone (PES) and polysulfone (PSU) (Chin et al, 2006 Molinari et al, 2000). This can be attributed to the fact that PES and PSU contain sulfone groups which are highly sensitive to UV light. Other membranes exhibiting low UV resistance are those made of polypropylene (PP), polyacrylonitrile... 

Polymer membranes have low proton conductivity until they contain enough proton conductive functional groups, e.g., sulfonic acid, in polymer backbones or side chains. Sulfonic acid in polymer chains shows a high dissociation constant, resulting in high proton conductivity in aqueous environments compared with other acids therefore, most DMFC membranes use the sulfonic moiety as the proton transfer medium. Sulfonation methods of polymer membranes are generally classified as post-sulfonation in the presence of polymers and in situ sulfonation through co-polymerization of sulfonated monomers and nonsulfonated monomers. 

The first type of hydrocarbon membrane for fuel cell applications was the sulfonated polystyrene-divinylbenzene co-polymer membranes equipped for the power source in NASA s Gemini space flights, but the sulfonated polystyrene had low chemical stability for long-term applications, because the proton on the tertiary carbons and benzylic bonds are easily dissociated in an oxygen environment forming hydroperoxide radicals. Since a styrene monomer is easily co-polymerized with other vinyl monomers via radical polymerization methods, various styrenic polymers were researched intensively. Two commercial polystyrene-based/related membranes are available BAM (Ballard), and Dais Analytic s sulfonated styrene-ethylene-butylene-styrene (SEBS) membrane. Dais membranes are produced using... 

The basic concept to use block co-polymer for the application to the DMFC is that ordered hydrophilic/hydrophobic phase separations offer a route for the selective transport of proton ions with reduced methanol crossover in the hydrophilic domains, because block co-polymers can be selectively sulfonated using post-sulfonation methods, and the block co-polymers can be verified over a wide range of structures during anionic polymerization. For example, methanol transport behaviors of a triblock co-polymer ionomer, sulfonated poly(styrene-isobutylene-styrene) (S-SIBS), were compared with Nafion to determine whether the sulfonated block co-polymer could serve as a viable alternative membrane for application to the DMFC [62]. The S-SIBS membranes showed approximately 5-10 times more methanol selectivity than that of Nafionll , although the S-SIBS membranes exhibited low conductivity compared with Nafion 117. 

From the results of intensive research, a number of polymer membranes were reported on high proton conductivity, low methanol permeation, and water uptake but many researchers assert that additional properties should be considered. The additional properties would be (1) the relation of molecular weights and mechanical properties (e.g., tensile modulus in dried and wet states) (2) dependence of acid treatment methods after sulfonation on proton conductivity (3) introducing optimized way for membrane electrode assembly (MEA), such as proper binders for new developed polymer membranes, pressure range, temperature, and catalyst layer configurations [22,65]. 

Stephen J. Paddison received a B.Sc.(Hon.) in Chemical Physics and a Ph.D. (1996) in Physical/Theoretical Chemistry from the University of Calgary, Canada. He was, subsequently, a postdoctoral fellow and staff member in the Materials Science Division at Los Alamos National Laboratory, where he conducted both experimental and theoretical investigations of sulfonic acid polymer electrolyte membranes. This work was continued while he was part of Motorola s Computational Materials Group in Los Alamos. He is currently an Assistant Professor in the Chemistry and Materials Science Departments at the University of Alabama in Huntsville, AL. Research interests continue to be in the development and application of first-principles and statistical mechanical methods in understanding the molecular mechanisms of proton transport in fuel-cell materials. 

---