Perfluorocarboxylate membranes

The self-diffusion coefficient of sodium ion is plotted in Fig. 11 vs. the reciprocal of absolute temperature for the perfluorosulfo-nate and perfluorocarboxylate membranes which were discussed in Section Here though, the membrane environment con-... 

Figure 13. Na self-diffusion flux in a perfluorocarboxylate membrane and membrane Na concentration vs. solution pH, 5 M NaCl solution environment, at 80°C. (Ref. 149 reprinted by permission of the publisher. The Electrochemical Society, Inc.)...

A novel perfluorocarboxylate membrane named Flemion was developed by Asahi Glass Co., Ltd. in 1975, resulting from its long experience both with manufacture of various fluorochemicals and with electrodialysis using membranes. 

As a result, the perfluorocarboxylated membrane with a high ion exchange capacity attains a high fixed ion concentration, which is defined as milliequivalent of carboxylic acid group per a gram of absorbed water in the membrane. 

Thus, high permselectivity combined with high conductivity is the outstanding feature of the perfluorocarboxylate membrane. 

The transniembrane transport of sodium is coupled with co-electro-osmotic transport of water which dilutes the caustic soda formed. Perfluorocarboxylic membranes can be substituted by perfluorosulphonic membranes for reducing the water transport. 

S. Diffusivity. The self-diffusion coefficient values for Na" " are presented in Fig. 4.8.14 for perfluorocarboxylate and perfluorosulfonate membranes [64,65]. These data show the self-diffusion coefficient to be lower in NaOH and also in NaCl solutions in the perfluorocarboxylate membranes compared to the same in the perfluorosulfonate membranes. This behavior reflects the high activation energy [65] for the perfluorocarboxylate membranes, as shown in Fig. 4.8.15. 

In perfluorocarboxylate membranes, the apparent diffusion coefficient of Na in NaOH is five times larger than that in 3.5N NaCl, while the diffusion coefficient values in NaCl and NaOH are similar, at about 10 cm s , in aqueous solutions. 

Thus, the perfluorosulfonate membranes provide low caustic efficiencies at high caustic strengths, while the perfluorocarboxylate membranes offer high current... 

The process was started in the early 1970s with development of the perfluorosul-fonate membrane Nation by DuPont [130]. In 1975, a perfluorocarboxylate membrane capable of producing 35 wt% caustic soda became available, from Asahi Glass in Japan [131]. In 1978 the first two-layer membrane was developed, with low electrical resistance and high current efficiency [132]. 

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