Perfluorosulfonic acid proton-conducting

This review will outline the materials requirements for advanced alternative proton exchange membranes for fuel cells, assess recent progress in this area, and provide directions for the development of next-generation materials. The focus will be on the synthesis of polymeric materials that have attached ion conducting groups. State-of-the-art Nation and its commercially available perfluorosulfonic acid relatives will initially be discussed. Other chain-growth co-... 

The PEM generally consists of polytetrafluoroethylene chains with hydrophilic perfluorosulfonate side groups. Water molecules within the system agglomerate in the vicinity of hydrophilic groups (i.e., sulfonic acid groups) and form hydrophilic clusters. A network of these clusters forms passages for proton conduction within PEM, which is... 

Until recently (i.e., till early 1990s), most of the efforts to develop DMFCs has been with sulfuric acid as the electrolyte. The recent success with a proton conducting membrane (perfluorosulfonic acid membrane) in PEMFCs has steered DMFC research toward the use of this electrolyte. The positive feature of a liquid feed to a DMFC is that it eliminates the humidification subsystem, as required for a PEMFC with gaseous reactants. Another positive point is that the DMFC does not require the heavy and bulky fuel processor. Two problems continue to be nerve-wracking in the projects to develop DMFCs (1) the exchange current density for methanol oxidation, even on the... 

Solid Polymer Electrolyte Fuel Cell Here, there is no apparent liquid solution, or high-temperature ionic conductor. The usual ionic solution between the electrodes is replaced by a well-humidified membrane made of a perfluorosulfonic acid polymer that conducts protons. 

Fig. 1 shows proton conductivity for the perfluorosulfonic acid membranes, measured at 300 K in air by the DC resistance measurement, after gamma-ray irradiation at the several doses up to 414 kGy. The conductivity was calculated from the applied voltage and the measured current and dimension of the polymers. It can be seen in Fig.l that the conductivity increases with increasing the dose. The conductivities at 300 K in air atmosphere rapidly increased until about 50 kGy, and achieved to be higher by about three orders of magnitude than that of the unirradiated one. 

Fig. 3 shows temperature dependences of conductivities in vacuum before and after heating the perfluorosulfonic acid membrane at the dose of 14 kGy to 393 K. The conductivities after heating decreased at higher temperature and reached to that for the unirradiated one at lower one. Particularly, it was found that the new protonic conduction process at lower temperature disappeared by heating. Moreover, it was confirmed that the conductivity increased again when was exposure to air at room temperature for 20 days. The conductivity depends on humidity in the enviroiunents and, namely, contents of water and hydrogen in the membrane. The modification for the absorption characteristic of water on the topmost surface greatly contributes to the new conduction mechanism at lower temperature. 

Kreuer et al. [25] investigated the membrane properties, including water sorption, transport (proton conductivity, electro-osmotic water drag and water diffusion), microstructure and viscoelasticity of the short-side-chain (SSC) perfluorosulfonic acid ionomers (PFSA, Dow 840 and Dow 1150) with different lEC-values. The data were compared to those for Nafion 117, and the implications for using such ionomers as separator materials in direct methanol and hydrogen fuel cells discussed. Tire major advantages of PFSA membranes were seen to be (i) a high proton conductivity. 

A characteristic feature of alternative membranes is that they nearly always exhibit a lower proton conductivity compared to Nafion for a similar ion content. The ionic conductivity can be improved by increasing the ion-exchange capacity (lEC) of the constituent polymer(s) but mechanical strength is frequently sacrificed firstly, in the dehydrated state because of the high ionic content, and secondly in the hydrated state due to excessive swelling [43]. Moreover, virtually all alternatives to perfluorosulfonic acid... 

While a munber of alternative polymer membranes have been developed. Nation is still considered the benchmark of proton conducting polymer membranes, and has the largest body of research hterature devoted to its study. Alternative polymer membranes are almost invariably compared to Nation . Nation is a free radical initiated copolymer consisting of crystaUiz-able, hydrophobic tetrafluoroethylene and a perfluorinated vinyl ether terminated by perfluorosulfonic acid. Nation 117 possesses an equivalent weight of 1100 (EW = mass of dry ionized polymer (g) in the protonic acid form that would neutralize one equivalent of base). Thus, there are 13 perfluoro-methylene groups (-CF2-) ( = 6.5) between pendent ionic side chains. 

Perfluorosulfonic acid (PFSA) membranes as shown in Fig. 1 were first developed for fuel cells by DuPont as Naflon and installed into the Biosatellite spacecraft in 1967 [1,2]. Various types of PFSA polymers, such as Flemion , Aciplex , and Dow membrane, were developed subsequently. They have excellent chemical stability, high proton conductivity, and high water diffusivity in a wide range of temperatures, brought about by the nature of fluorinated compounds and these non-cross-linked structures [3-5]. 

As a proton-conducting polymer, Nafion (a perfluorosulfonic acid polymer) has high ionic conductivity and has also been investigated as an electrolyte for different types of solid-state ESs [852-858]. It has been found that high scan rates could be achieved for ESs with Nafion electrolytes [854,855,859]. 

In general, it is known that the proton conductivity does not directly correlate with either water uptake or lEC for any of the polymers applicable in fuel cell technology. Sulfonated hydrocarbon polymers usually have greater water uptake than the perfluorosulfonic polymers in order to achieve the same proton conductivity, which is probably because the acid content in the hydrocarbon polymers is much lower... 

FIGURE 4.8.11. Variation of proton conductivity of perfluorosulfonic acid membranes with water content at 30 C. Dow A DuPont Nafion 117 (revised from the figure in [59]). 

C. Filipoi, D.E. Demco, X. Zhu, R. Vinokur, O. Conradi, R. Fechete, M. MoeUer, Water self-diffusion anisotropy and electrical conductivity of perfluorosulfonic acid/ SiOa composite proton exchange membranes, Chem. Phys. Lett. 554 (2012) 143—149. 

K.D. Kreuer, M. Schuster, B. Obliers, O. Diat, U. Traub, A. Fuchs, U. Klock, S.J. Paddison, J. Maier, Short-side-chain proton conducting perfluorosulfonic acid ionomers why they perform better in PEM fuel cells, J. Power Sources 178 (2008) 499-509. 

L. Ghassemzadeh, K.D. Kreuer, J. Maier, K. Mueller, Evaluating chemical degradation of proton conducting perfluorosulfonic acid ionomers in a Fenton test by sohd-state F NMR spectroscopy, J. Power Sources 196 (2011) 2490—2497. 

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