9 Transport Phenomena and Morphology Changes Associated with Nafion 390 CationExchange Membranes
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S. G . C U T L E R ICI Ltd, Mond Division, Research and Development Department, P. O. Box 7, Winnington, Northwich, Cheshire, England C W 8 4 D J
Electrolysis, for the production of alkali and chlorine, using Nafion 390 (a perfluoro sulfonic acid cation-exchange membrane) as a cell separator is described. The current efficiency of the membrane increases with the applied electrical field gradient. However, membrane transport phenomena at high current density (0.5 amps cm ) are unstable and the selectivity decreases over a period of several days. The use of low-angle x-ray diffraction and electron transmission microscopy indicate that this decrease in selectivity correlates with a dramatic transition in membrane morphology, resulting in a thermodynamically more stable expanded form. The hydrated membrane as supplied contains ion clusters of about 50 Åin diameter; in the expanded form, which involves gross molecular rearrangement, ion clusters are as large as 100 Å. -2
T ^ h e discovery of the ion-exchange properties of phenol formaldehyde - - resins i n 1935 by Adams and Holmes ( I ) led to the synthesis of the first high capacity ion-exchangers. This discovery, coupled with subsequent advances i n polymer science and technology, led to the availability of many types of hydrocarbon-based ion-exchange membranes i n the early 1950's. This era also witnessed the emergence of numerous patents on the use of ion-exchange membranes i n various electrochemical processes that were of industrial significance. O f particular interest to the chlor-alkali industry was the use of cation-exchange membranes as cell separators i n brine and water electrolysis. A diagram of such a cell is 1
0-8412-0482-9/80/33-187-145$05.00/0 © 1980 American Chemical Society
Eisenberg; Ions in Polymers Advances in Chemistry; American Chemical Society: Washington, DC, 1980.
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146
IONS I N P O L Y M E R S
shown i n Figure 1. A saturated sodium chloride solution is fed to the anode compartment where chloride ions are oxidized to chlorine gas, while water is reduced at the cathode to yield hydroxyl ions and hydrogen gas. T o restore electroneutrality, sodium ions are transported selectively i n the electrochemical field gradient across the cation-exchange membrane from the anode to the cathode chamber. Ideally the membrane should be 100% cation permselective, therefore excluding any hydroxyl ion trans port; but i n practice this is not the case and current efficiencies are always less than 100%. This current inefficiency is represented by the reaction of hydroxyl ions with chlorine. Patent applications for this method of chlor-alkali production appeared as early as 1949 (2). The advent of D u Pont's inert perfluorosulfonic acid membranes, Nafion, in the late 1960's made chlor-alkali production in a membrane cell a realistic possibility. The basic structure of D u Pont's fluoro ionomer is shown (CF CF ) (CF CF) I Ο 2
2
n
2
m
CF
\
3
/ [CF CF—0]^—CF CF S0 H 2
2
2
3
Copolymers of the type shown can be fabricated into large sheets and are characterized by their equivalent weight, that is, the weight i n grams of polymer i n the acid form required to neutralize one equivalent of base.
BRINE IN
WATER IN
CATHODE
Ο
CAUSTIC OUT
BRINE OUT CATION-EXCHANGE MEMBRANE
Figure 1.
Cation-exchange membrane
Eisenberg; Ions in Polymers Advances in Chemistry; American Chemical Society: Washington, DC, 1980.
Downloaded by CORNELL UNIV on October 30, 2016 | http://pubs.acs.org Publication Date: June 1, 1980 | doi: 10.1021/ba-1980-0187.ch009
9.
CUTLER
147
Nafion Cation-Exchange Membranes
The work described relates to the study of transport phenomena and morphology changes associated with Nafion 390 when used i n an electro chemical cell. This membrane is a laminate of two equivalent weights of copolymer, 1500 and 1100, supported by a r a y o n / P T F E mesh. W h e n installed i n a cell, the 1500 equiv wt faces the cathode compartment. A rigorous interpretation of transport phenomena associated with Nafion 390 during electrolysis is not possible, since no adequate theory exists for systems so far from equilibrium and under such high applied forces. In addition, a further complicating factor is that observed trans port phenomena result from the combined effects of both membranes in the laminate. However, as in the theory of irreversible thermodynamics (3,4), each flux / across the membrane may depend on every applied force X . Thus the flux of species i across the membrane may be written as Ji =
Σ
(1)
LijcXjc
allfc where L = f(X ) are called phenomenological coefficients and represent the interaction of species i with k. In the case of the electrochemical cell shown in Figure 1, the main force acting i n the system is the applied field gradient. The current efficiency for electrolysis is given by ik
k
CE — ^
Na
