Membranes membrane development Nafion

Following a period of slack, decisive improvements were made after 1990 in the area of PEMFCs. Modem models now achieve specific powers of over 600 to 800 mW/cm while using less than 0.4 mg/cm of platinum catalysts and offering a service fife of several tens of thousands of hours. These advances were basically attained by the combination of two factors (1) using new proton-exchange membranes of the Nafion type, and (2) developing ways toward much more efficient utilization of the platinum catalysts in the electrodes. 

DuPont s perspective on membrane development will be outlined. The expansion of the facilities at the Nafion Customer Service Laboratory will also be described. This expansion has been undertaken in support of DuPont s commitment to increase the understanding of chlor-alkali technology and ensure continuous improvement of DuPont s membranes. 

The current state-of-the-art proton exchange membrane is Nafion, a DuPont product that was developed in the late 1960s primarily as a permselective separator in chlor-alkali electrolyzers. Nation s poly(perfluorosulfonic acid) structure imparts exceptional oxidative and chemical stability, which is also important in fuel cell applications. 

Concentration Method. The concentration procedure that was developed and evaluated was a RO-Donnan dialysis system (4). The initial objective during method development was to conduct membranescreening tests to evaluate the suitability of various RO and ion-exchange membranes. The four membranes considered for final evaluation on the basis of solute rejection, chlorine stability, and artifact production were the cellulose acetate and FT-30 (Film Tec) RO membranes, the Nafion cation-exchange membrane, and the ION AC MA 3475 anion-exchange membrane. 

Subsequent to these early developments of alloy electrocatalysts in the PAFC technology have been attempts to use the same in pefluorinated sulfonic acid fuel cells (solid-state membranes such as Nafion from Dupont, Dow, Asahi, and others). Yeager et al. [19] have reviewed the effect of different electrolytes on the ORR electrocatalysis. The summary of this work is that the solid-state perfluorinated acid environment offers significant advantages over phosphoric acid. These are... 

The electrocatalytic oxygen reduction in DMFC systems requires the development of highly selective electrodes in the presence of methanol. Present electrolyte membranes based on Nafion 117 are permeable to methanol (crossover effect), which depolarizes the platinum cathode [81,82]. For this reason, our strategy, some years ago, was to produce electrocatalytic materials from the thermal decomposition of some neutral transition-metal carbonyl compounds in the presence... 

The development of new polymeric materials for polymer electrolyte fuel cell is one of the most active research areas, aiming at the new energy sources for electric cars and other devices. The mainstream of the material research for fuel cell is perfluoroalkyl sulfonic acid membranes such as Nafion, Acipex, and Flemion. The most well-known one is Nafion of Du Pont, which is derived from copolymers of tetrafluoro-ethylene and perfluorovinyl ether terminated by a sulfonic acid group.Protons, when dissociated from the sulfonic acid groups in aqueous environment, become mobile and the membrane becomes a proton conducting electrolyte membrane. 

At the present time, the Nogoya Institute in Japan is making progress developing a new glass based electrolyte that is much less expensive than fluoropolymer membranes, but just as durable. In the near future, these membranes may replace Nafion and SPEEK in PEM cells. 

In April 1975, Asahi Chemical started operation of a membrane chlor-alkali plant with a capacity of 40,000 MT/Y of caustic soda using Nafion perfluorosulfonic acid membrane. In 1976, this membrane was replaced by perfluorocarboxylic acid membrane developed by Asahi Chemical. The total caustic production capacity of plants based on Asahi Chemical s membrane chlor-alkali technology using perfluorocarboxylic acid membrane will reach 520,000 MT/Y in 1982, at seven locations in various countries. 

At this point, we do not know which of several approaches is most promising. Thus, our membrane development efforts involve (1) a full-fledged effort to explore approaches involving polymer synthesis and development, as well as implementation of new carrier media to replace the function of water in Nafion, and (2) a study of proton transfer dynamics. We are using theoretical approaches to explore specific possibilities for new acid group types or for... 

Recently, Dahr [1], Stonehart [2] and Watanabe [3] have made an attempt to reduce the humidification constraints in solid polymer electrolyte fuel cells (SPEFCs) by using modified perfluorosulfonic membranes. A recast Nafion film sandwiched between the two electrodes was first proposed by Dahr [1] for the realization of an internally humidified SPEFC. Stonehart [2] suggested the inclusion of small amounts of silica powder into the recast film in order to retain the electrochemically produeed water inside the membrane. Watanabe et al [3] have tried to exploit the H2/O2 crossover through the membrane to produce a chemical recombination to water on small Pt clusters inside the membrane. All of these membranes were operated with H2/O2 at 80°C and allowed the development of systems without assisted humidification or with near ambient humidification. 

The membrane in a membrane fuel cell fulfils several important functions as stated in the introduction. Nafion was the first commercially available membrane, which lead to a breakthrough in fuel cell technology. Today, various companies are engaged in membrane development especially for this purpose, aiming at improved material properties. The goals are less sensitivity towards elevated temperature and dry operation, better chemical and mechanical stability and reduced methanol crossover for DMFC operation. A significant improvement of the mechanical stability was achieved by incorporation of a PTFE porous sheet as mechanical support for the membrane material [13,14]. 

At high current densities, the ohmic resistance of the membrane still affects the properties of the fuel cell. For improved performance, thinner membranes of Nafion 112, 50 pm thick, were introduced in the place of the standard Nafion 115 membranes (100 pm). Even thinner membranes have been developed (Nafion 111,25 pm), but fuel cells with such membranes occasionally failed because of small membrane defects allowing the gases to mix. 

Several reviews on membranes for DMFC fuel cells have been published in the last decade [1-9], starting with that by Kreuer [1], discussing the differences between Nafion and sulfonated polyether ketone membranes. According to Fig. 6.1, reviews published till 2006 [1 ] cover only one third of the ionomeric membranes currently developed for DAFC. More recent reviews deal with polyimide ionomer membranes [5], composite membranes for high temperature DMFC [6], non-perfluorated sulfonic acid membranes [7], modified Nafion membranes [8], and hybrid membranes [9-11]. 

Recently, polyimides have drawn considerable attention for the development of membranes for fuel cells. In polymer membrane fuel cells, nafion is widely used as membrane. However, the nafion membrane cannot withstand high temperature (>80 °C), so research on the development of polyimide based fuel cell membranes has started [169-173]. Polyimides also find applications in the development of gas separation membranes because of their compact ring structure and less free volume, which restricts the passage of gases. 

More recently, Tigelaar et al. have developed an alternative polymer membrane to Nafion that contains rigid sulfonated aromatic backbones linked with diamine-terminated poly-(ethylene oxide) (PEO) oligomers. This polymer was developed to increase the amount of ionic liquid in the membrane, where in Nafion, the perfiuorinated nature was thought to be reducing the uptake. The polymer membranes were soaked with a 1 1 mixture of a PIL and water, where the PILs trialed contained the imidazolium cation with either sulfate, triflate, or TFSI anions. The highest conductivity obtained with these polymer membranes was 50 mS/cm at 150 °C with imidazolium—CF3SO3H. ... 

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