DuPont’s Nafion

This review summarizes the recent works on syntheses of solid superacids and their catalytic action, including Lewis acids and liquid superacids in the solid state, as discussed in Sections Il-IV. Sections VI and VII describe new types of solid superacids we have studied in this decade sulfate-supported metal oxides and tungsten or molybdenum oxide supported on zirconia. Perfluorinated sulfonic acid, based on the acid form of DuPont s Nafion brand ion membrane resin, is also gaining interest as a solid superacid catalyst Nafion-H-catalyzed reactions are reviewed in Section V. 

Regenesys uses DuPont s Nafion (Section 6.1.7) as the perm-selective sodium ion transfer membrane, separating the two half cells. Figure 2.1. Diffusion of sodium ions in the concentration difference across the Nafion membrane is one of the irreversibilities of the system. The low-cost plastic (e.g. polyethylene) tanks and pipework are treated with fluorine to provide bromine resistance, and are able to operate with, and contain, both electrolytes at ambient temperature. 

It is assumed that DuPont s Nafion membranes are used in about 80% of all pubhcations on PEMFCs. The simple reason is that this was the only commercially available material for long time. Therefore, it is not possible to present here a complete... 

Another type of solid superacid is based on perfluorinated resin sulfonic acids, such as the acid form of DuPont s Nafion resin, a copolymer of a perfluorinated epoxide and vinylsulfonic acid, or higher perflu-oroalkanesulfonic acids such as perfluorodecanesulfonic acid, CF3(CF2) 03H. Such solid catalysts were found to be very efficient in alkylation of aromatic hydrocarbons and other Friedel-Crafts reactions. A comprehensive review is available on the application of Nafion-H in organic catalysis. ... 

Perfluorinated polyethers have also gained importance as actively functional materials. Ionic polymer membranes (e.g. DuPont s Nafion ) based on sulfonic acid-derivatized perfluoropolyethers have been used for nearly 30 years as ion-con-ducting membranes in chloralkali electrolysis cells, replacing the large amounts of toxic mercury used until then in the classic Castner-Kellner cells (Scheme 4.8.). One of the earliest applications of Nafion was as a membrane in the hydrogen-oxygen fuel cells which powered the Apollo spacecraft carrying the first men to the moon. 

One recent breakthrough in membrane technology occurred when PolyFuel, in Mountain View, CA, produced a hydrocarbon polymer membrane with improved performance and lower costs than the current per-fluorinated membranes. This cellophane like film has performed better than more common perfluorinated membranes, such as Dupont s Nafion material. The hydrocarbon membrane can also operate at higher temperatures, of up to 95°C, which allows the use of smaller radiators to dissipate heat. It also lasts 50% longer, while generating up to 15% more power and operating at lower humidity levels. 

Cation, anion, and water transport in ion-exchange membranes have been described by several phenomenological solution-diffusion models and electrokinetic pore-flow theories. Phenomenological models based on irreversible thermodynamics have been applied to cation-exchange membranes, including DuPont s Nafion perfluorosulfonic acid membranes [147, 148]. These models view the membrane as a black box and membrane properties such as ionic fluxes, water transport, and electric potential are related to one another without specifying the membrane structure and molecular-level mechanism for ion and solvent permeation. For a four-component system (one mobile cation, one mobile anion, water, and membrane fixed-charge sites), there are three independent flux equations (for cations, anions, and solvent species) of the form... 

It is assumed that DuPont s Nafion membranes are used in about 80% of all pubhcations on PEMFCs. The simple reason is that this was the only commercially available material for long time. Therefore, it is not possible to present here a complete overview of performance data with Nafion or other membranes. Furthermore, the performance depends strongly on the conditions used as well as MEA preparation (catalyst loading, etc.). As a consequence, only the performance data from the membrane manufacturer itself are reported. Figme 21.20 exhibits performance reports by DuPont in 2002 for a three-layer MEA for neat hydrogen and reformate operation. 

After that, advances in DMFC mostly hibernated until 1992, when DuPont s Nafion solid polymer electrolyte membrane was found to be an excellent proton conducting media [26]. Thus, in 1994, a collaborative work involving the Jet Propulsion Laboratory, University of Southern California, and Giner Inc., demonstrated a Nafion-based DMFC [27], which could deliver power outputs up to 150 mW.cm at 90 °C. 

Around the same period, Asahi Chemical started a pilot plant operation with ion-exchange membrane cells, using DuPont s Nafion . In 1975, a commercial plant was commissioned, producing about 10 tons of caustic/day, at Nobeoka in Japan. 

It is mainly the PEM that distinguishes a PEM fuei ceii from aii other types of fuel cells. As its name implies, a PEM has the capability of transporting protons. It is typicaiiy made of a solid ionomer with acidic groups such as sulfonic acid (-SO3H) at the end of the polymer side chains. Polystyrene sulfonic acid is one such ionomer, and it was used as the PEM in the early days of the PEM fuel cell development around the 1960s. However, since the PEM fuel cell environment is warm, corrosive, and oxidative (at cathode), an ionomer with higher chemical and electrochemical stability is required. State-of-the-art PEMs are made of perfluorinated polysulfonic acids, and include DuPont s Nafion . 

Acidic ion-Exchange Resins Cataiysts. Commercial solid acidic ion-exchange resins have been demonstrated to catalyze the carbonylation of alkenes and alcohols, when operated under proper conditions (65). The resins comprise various commercial sulfonated styrene-divinylbenzene resins, both in gel and in microreticular forms Dupont s Nafion NR50 and its Nafion-silica composite SAC 13 as well as Degussa s sulfonated polysiloxane resin Deloxan ASP. 

In order to increase the proton conductivity, a lower EW ionomer can be made. However, when the EW is too low (e.g., less than 700), the mechanical strength of the membrane becomes unacceptable, especially after the membrane is fully hydrated, which will in turn shorten the membrane lifetime. The proton conductivity and the mechanical strength of a membrane also depend on the side chain length and structure. The PFSAs made by Dow and 3M have shorter side chains than DuPont s Nafion, as shown in Pigure 1.3. 

In a comparison with DuPont s Nafion perfluorinated membrane material whose operation is said to require a humidity level of at least 50% and a temperature below 90 °C, Polyfuel states that its product will operate at temperatures of up to 95 °C and humidity levels as low as 35%. 

Other imaginative uses of rare earths include the reduction of alkenes, aldehydes and ketones with intermetalhc compounds containing absorbed hydrogen, viz. LaNijHg (Imamoto et al., 1984d). The use of duPont s Nafion polymer as a support for Cr(III) and Ce(IV) reagents useful in the oxidation of alcohols has also been reported (Kanemoto et al., 1984). Such conversions also involved the use of t-butylhydroperoxide or sodium bromate as cooxidant. 

The success of DuPont s Nafion spurred the development of other polymeric materials with similar chemical architecture. The most notable material developments have been the Dow experimental membrane (Dow Chemicals), Flemion (Asahi Glass), Aciplex (Asahi Kasei), as well as Hyflon Ion and its most recent modification Aquivion (SolviCore). In addition to excellent ionic conductivity, materials of the PFSA family, illustrated in Figure 2.2, exhibit exceptional stability and durability in highly corrosive acidic environments, owing to their Teflon-like backbone (Yang et al., 2008 Yoshitake and Watakabe, 2008). 

Proton-exchange membrane fuel cells (PEMFC)—use solid-polymer proton-conducting membrane electrolyte at temperatures generally ranging from ambient to 90°C. Today s technology primarily uses the trifluoromethanesulfonic-acid-based electrolyte membrane, such as DuPont s Nafion . 

Abstract There have been numerous studies on modifying DuPont s Nafion (a perfluorosulfonic acid polymer) in order to improve the performance of this membrane material in a direct methanol fuel cell. Modifications focused on making Nafion a better methanol barrier, without sacrificing proton conductivity, so that methanol crossover during fuel cell operation is minimized. In this chapter, a brief literature survey of such modifications is presented, along with recent experimental results (membrane properties and fuel cell performance curves) for (1) thick Nafion films, (2) Nafion blended with Teflon-FEP or Teflon-PFA, and (3) Nafion doped with polybenzimidazole. 

The most widely studied fuel-ceU membrane is DuPont s Nafion , a copolymer of tetrafluoroethylene and perfluoro(4-methyl-3,6-dioxa-7-octene-l-sulfonic acid). Nafion is the membrane material of choice for most proton-exchange membrane fuel cells that operate at a temperature <80 °C. While Nafion offers high conductivity combined with exceptional chemical and mechanical stability [3], it suffers from several critical drawbacks. When used in a direct methanol fuel cell, Nafion shows significant methanol leakage (crossover from the anode to the cathode) with the resultant reduction in fuel-ceU performance. To overcome this shortcoming the methanol concentration in the anode feed is usuaUy reduced to 0.5-2.0 M, which necessitates... 

DuPont s Nafion is the most advanced commercially available proton conducting polymer material, which is produced in membrane form with thickness between 25 and 250 pm. Nafion is the electrolyte against which other membranes are judged and is in a sense an Industrial Standard . It is a copolymer of tetrafluoroethylene (TFE), and perfluoro (4-methyl-3,6-dioxa-7-octene-l-sulfonyl fluoride) or vinyl ether , as shown in Fig. 3. The methods of creating and adding the side chains are highly complicated and the process involving many steps is proprietary. 

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