Membranes for chlor-alkali cells

In 1972, when the main thrust for these then-new materials was the development of efficient polymer membranes for chlor-alkali cells, the F. I. du Pont Co. reported much information, mostly in the form of product literature, in the form of empirical equations regarding the water mass and volume uptake of sulfonate form Nafion as a function of FW (in the range 1000—1400), some alkali metal counterion... 

Significant advances have been made in recent years to develop high-performance membranes for chlor-alkali cells by a judicious... 

The development of membranes for chlor-alkali cells is, however, a continuing activity and two problems remain to be resolved (i) OH transport becomes... 

J.T. Keating, Sulfate Deposition and Current Distribution in Membranes for Chlor-Alkali Cells. In F. Hine, W.B. Darlington, R.E. White, and R.D. Vaijian (eds), Electmchemical Engineering in the Chlor-Alkali and Chlorate Industries, PV 88-2, The Electrochemical Society Pennington, NJ (1988), p. 311. 

This is evidenced by the amount of literature on ionomers and by the appearance of two monographs devoted to the subject (J, ). Most of the research effort on the ionomers has focused on only a small number of materials, notably ethylenes (3-9 ), styrenes (10,11), rubbers (12-16) and recently aromatic (17) and fluorocarbon-based ionomers (18). The last material is known for its high water permeability and cation permselectivity. Because of its unique properties, it has been employed as an ion-exchange membrane in chlor-alkali cell operations in electrochemical industries. Perfluorinated ion-exchange membranes are the subject of the present chapter. 

Other perfluorinated cation exchange membrane materials have also been produced for chlor-alkali cell and other applications. These are the Flemion membrane products (Asahi Glass Co. 

Electrochemical processes require feedstock preparation for the electrolytic cells. Additionally, the electrolysis product usually requires further processing. This often involves additional equipment, as is demonstrated by the flow diagram shown in Figure 1 for a membrane chlor-alkali cell process (see Alkali AND chlorine products). Only the electrolytic cells and components ate discussed herein. 

Fig. 1. Flow diagram for chlor-alkali production by a membrane cell process.

Electric energy is the predominant cost in the manufacture of chlorine and is the driver for most of the technical progress in the chlor-alkali industry. The busiest areas of development over the past 20 or 30 years have been related to reductions in energy consumption. Approximately 60% of the papers presented in this book deal with improvements in chlor-alkali cell internals, namely the anolyte/catholyte separator (primarily membranes) and the electrodes. 

Monopolar electrodes have a direct electrical connection with an external power supply. This requires the distribution of current over the total area of one monopolar electrode, collecting the current from the other monopolar electrode for conduction to the next cell through intercell busbars. Monopolar cells operate at low voltages, and may require high amperages. Industrial circuits of cells may consist of one hundred or more monopolar cells in series. Monopolar electrodes are used in some membrane chlor-alkali cells (Figs. 4 and 5), fluorine cells (Fig. 6), and in metal electrowinning cells (Fig. 7). 

Equilibration with water vapor Substantially less work was done in this area in the early phase of research on PFSAs because of previous emphasis on membranes which would be in contact with liquid water or aqueous solutions (e.g., for chlor-alkali technology). However, water supplied from the vapor phase could be a principal mode of external hydration of the membrane in a PEFC, particularly hydration of the anode side, and thus it is an important focus of study in fuel cell R D. The shape of the sorption isotherms shown in Fig. 29 (a) and (b) is generic for ion-exchange polymers. With increasing PhsO. water is sorbed in two steps as evidenced by the sorption isotherm ... 

Perfluorinated carboxylate membranes were introduced about seven years ago. These membranes can be synthesized by a variety of methods or by various chemical conversions from the Nafion polymer.Composite membranes which contain both sulfonate and carboxylate functional groups have also been produced (see Section IV.l for more details). These carboxylate membranes have been widely employed in the advanced membrane chlor-alkali cells. This major chemical technology is in the process of being revolutionized by the use of these materials, a remarkable accomplishment for such a small group of polymers. ... 

Electroosmotic effects also influence current efficiency, not only in terms of coupling effects on the fluxes of various species but also in terms of their impact on steady-state membrane water levels and polymer structure. The effects of electroosmosis on membrane permselectivity have recently been treated through the classical Nernst-Planck flux equations, and water transport numbers in chlor-alkali cell environments have been reported by several workers.Even with classical approaches, the relationship between electroosmosis and permselectivity is seen to be quite complicated. Treatments which include molecular transport of water can also affect membrane permselectivity, as seen in Fig. 17. The different results for the two types of experiments here can be attributed largely to the effects of osmosis. A slight improvement in current efficiency results when osmosis occurs from anolyte to catholyte. Another frequently observed consequence of water transport is higher membrane conductance, " " which is an important factor in the overall energy efficiency of an operating cell. 

Membranes can be characterized by their structure and function, that is how they form and how they perform. It is essential that the cation exchange membranes used in chlor-alkali cells have very good chemical stability and good structural properties. The combination of unusual ionic conductivity, high ionic selectivity and resistance to oxidative hydrolysis, make the perfluorinated ionomer materials prime candidates for chlor-alkali membrane cell separators. 

Only after viewing the membrane as a thin film semiconductive phase can one begin to seriously evaluate its potentialities. It is a multidimensional problem, and in the chlor-alkali cells the water transport is controlled by brine concentration while caustic strength controls the cathode efficiency. The membrane provides a low energy pathway for the phase change and separation process. 

Surface treatment has also been employed to generate membranes with improved hydroxide ion rejection capability for chlor-alkali applications. In this procedure, one surface of a sulfonyl fluoride XR resin film is treated with an amine such as ethylene-diamine. After hydrolysis, a thin barrier layer of weakly acidic sulfonamide exchange sites is formed. When this treated surface faces the cathode solution, improved hydroxide rejection is realized in a membrane chlor-alkali cell. 

Figure 9. Current efficiency vs. NaOH catholyte concentration for Nafion 227 membrane in a chlor-alkali cell (34). Conditions current density, 31 A/dm2 temperature, 85° C anolyte concentration, 4.4 N NaCl cell voltage, 4.6 V.

Hora, C.J. Maloney, D.E. "Nafion Membranes Structured for High Efficiency Chlor-Alkali Cells", presented at the 152nd National Meeting of The Electrochemical Society, Inc., Atlanta, Ga., October 10-14, 1977. 

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