Hydrocarbon membranes sulfonated

Polyarylenes, in particular different types of poly(arylene ether ketone)s, have been the focus of much hydrocarbon membrane research in recent years. - - With good chemical and mechanical stability under PEM fuel cell operating conditions, the wholly aromatic polymers are considered to be the most promising candidates for high-performance PEM fuel cell applications. Many different types of these polymers are readily available and with good process capability. Some of these membranes are commercially available, such as poly(arylene sulfone)s and poly(arylene... 

These compounds are synthesized by the copolymerization of tetrafluoroethylene and alkyl vinyl ether with sulfonyl acid fluoride. Preparation of sulfonyl acid fluoride takes place by pyrolysis of the respective oxide to give the olenfinated structure [7], The thermoplastic produced is extruded into a film. The sulfonyl fluoride (-S02F) group present in the thermoplastic is converted to sulfonate (-S()3 Na+) with NaOH. This is called the neutral form of Nafion, and it is converted to the acid for and cast into a thin film by heating in alcohol at 250°C. At around 650/m2, the Nafion membranes are rather expensive when compared to hydrocarbon membranes, which are also used. 

C.H. Park, C.H. Lee, M.D. Guiver, Y.M. Lee, Sulfonated hydrocarbon membranes for medium-temperature and low-humidity proton exchange membrane fuel cells (PEMFCs). Progress in Polymer Science 2011, 36(11), 1443-1498. 

Due to fluorination, PFSA materials are relatively more costly than non-fluorinated hydrocarbon membranes. The latter such as poly (ether ether ketone) and poly (arylene ether sulfone) have also been widely investigated for fuel cell applications. In addition to lower cost, some of them also have better dimensional stability with regard to hydration levels. The problem is their lower chemical stability in the fuel cell environment. 

Hydrocarbon Membranes for Polymer Electrolyte Fuel Cells, Fig, 5 Sulfonated poly(/ -phenylene)/poly(arylene ether ketone) block copolymer... 

Another typical kind of sulfonated hydrocarbon polymer, sulfonated polysulfone (sPSU), has also been imbibed with triazole [84, 85]. In a doped membrane of a base doping level of 8.3, the pulsed field magnetic gradient (PFG) NMR spectroscopy showed a self-diffusion coefficient of the triazole 4-5 times lower than that... 

Originally, sulfonated polymer membranes were used as an alternative to PFSA membranes for low-temperature operation, due to the former s lower cost. Recently, however, some—especially sulfonated hydrocarbon membranes—were found to be less dependent on humidity than were PFSA membranes if their associated H20/S03 ratios were low. This allowed them to have good conductivity at high temperatures, which is promising for high-temperature operation. 

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... 

Results from a long-term durabihty test of a hydrocarbon membrane show behavior opposite to that predicted. The sulfonated polyimide (SPI) membrane demonstrated 5,000-h lifetime when tested at 0.2 A cm, 80°C, and ambient pressure (Aoki et al. 2006a). Even if there is Nafion PFSA ionomer in the electrode, the SPI cell does not show any fluoride release, indicating that ionomer degradation in the catalyst layer is minimal. It must be stressed that SPI is 40 times less permeable to... 

Similar to pure hydrocarbon membranes discussed previously, both the aromatic rings and the SOj" groups are lost after the durability test (Yu et al. 2003). No preferential cleavage of sulfonic acid groups was observed. On the basis of XPS results, it also is found that chemical degradation during fuel cell operation occurs mainly on the hydrocarbon fraction of the radiation-grafted ionomer membranes (Nasef and Saidi 2002). Thus, the hydrocarbon fraction has less chemical stability than the fluorinated part. 

Keywords Block copolymers Hydrocarbon membranes Proton exchange membranes Sidechain functionalized polymers Sulfonated poly(arylene ether)s ... 

Nafion is an ionomer that has a hydrophobic tetrafluoroethylene (TFE) backbone and perfluoro alkyl ether (PFAE) side chains terminated with hydrophilic sulfonate salt [1 ], as shown in Fig. 1. It was introduced by DuPont in 1960 and has been used in a variety of applications such as an ion-selective membrane for the Qilor-Alkali process [5, 6], a water transport membrane for humidifiers, and as a strong acid catalyst [7]. Nafion was introduced as a polymer electrol34e membrane (PEM) in fuel cells in the early 1990s [4, 8]. Compared to hydrocarbon membranes, Nafion was chemically more robust in the fuel cell and the acid form has high proton conductivity. There have been many efforts to find alternative membrane materials... 

In sulfonated hydrocarbon polymers, the hydrocarbon backbones are less hydrophobic and the sulfonic acid functional groups are less acidic and polar. As a result, the water molecules of hydration may be completely dispersed in the nanostructure of the sulfonated hydrocarbon polymers. Both PFSA and sulfonated hydrocarbon membranes have similar water uptakes at low water activities, whereas at high relative humidity (100%) PFSA membranes have a much higher water uptake due to the more polar character of the sulfonic acid functional groups. The sulfonated aromatic polymers have different microstructures from those of PFSA membranes (Fig. 8) (Li et al., 2003). 

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