Cation-exchange membranes, perfluorosulfonic

Cation-exchange membranes, perfluorosulfonic acid ionomer, 125... 

The principle of operation is shown in Fig. 2. Chlorine gas is produced at the anode (especially optimized dimensionally stable anode) with an anolyte feed concentration of 14 wt % HCl. Anode and cathode are separated by a cation exchange membrane (perfluorosulfonic acid polymer, PFSA, e.g., Nafion of DuPont). The ODC is based on a conductive carbon cloth which operates simultaneously as a gas diffusion layer because a suitable material is incorporated. The oxygen reduction reaction (5) takes place in three-phase boundaries of a thin, porous catalyst layer on the surface. 

In 1973, Dupont began to commercialize their first perfluorosulfonic add cation exchange membrane, Nafion. Since then until now, Nafion has been attracting much attention because of its superb chemical and thermal stability, high ionic conductivity, excellent permselectivity and good mechanical strength. Many approaches have been proposed to use this unique material as a modifier of electrochemical electrode surfaces. 

Other perfluorosulfonate cation exchange membranes with similar structures have also been devel-... 

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... 

Z. Ogumi, Y. Uchimoto, M. Tsujikawa, K. Yasuda and Z. Takehara, Modification of ion-exchange membrane surface by plasma process. Part 2. Monovalent cation permselective membrane from perfluorosulfonate cation exchange membrane, J. Membr. Sci., 1990, 54, 163-174. 

R. Schlogl and F. Helfferich, Theory of exchange membrane potentials, Z. Elektro-chem, 1952, 56, 644-647 N. Ishibashi, T. Seiyama and W Sakai, Electrochemical studies on ion exchangers (Part 10) Mobilities of Ca+ and Cl- in the cation exchange membrane, Denki Kagaku (J. Electrochem. Soc. Jpn.), 1955, 23, 182-186 E.E. Boakye and H.L. Yeager, Water sorption and ionic diffusion in short side chain perfluorosulfonate ionomer membranes, J. Membr. Sci., 1992, 69, 155-167. 

Hydrogen sulfide and methane fluxes were measured at ambient conditions for 200 um perfluorosulfonic acid cation exchange membranes containing monoposltlve EDA counterions as carriers. Facilitation factors up to 26.4 and separation factors for H2S/CH up to 1200 were observed. The HjS transport Is diffusion limited. The data are well represented by a simplified reaction equilibrium model. Model predictions Indicate that H S facilitated transport would be diffusion limited even at a membrane thickness of 1 um. 

Figure 2. The structure of the perfluorosulfonic acid cation exchange membrane. The value of m=1 for membranes used in this study.

The diaphragm may be replaced by a cation exchange membrane which is suitable for high concentrated hydrochloric acid like Nafion (DuPont, perfluorosulfonic acid polymer, PFSA, see entry Chlorine and Caustic Technology, Membrane Cell Process ). This membrane has almost no usual porosity and is nearly exclusively permeable for ions including a hydration shell of some water molecules. Thus, product quality is significantly increased, process operation can be simplified, and cell voltage is reduced by about 0.3 V [1, 6]. However, the mechanical durability... 

SP (Vis) The redox indicatoi ferroin, tris(l,10-phenanthroline)Fe(II) is incorporated into the perfluorosulfonated cation exchange membrane Nafion and, together with optical fibers, a photodiode, and a light-emitting diode, is used for the construction of a redox optical sensor (optode)... 

Both perfluorosulfonate membranes and perfluorocarboxylate membranes are cation exchange membrane (cation Na ) and are based on perfluorocarbon polymer. However, the fixed group of perfluorosulfonate membranes is sulfonate pendant (-SO3-) while perfluorocarboxylate is carboxylate pendant (-COO-). This structure difference leads to different physicochemical properties. 

Some vinyl fluoride-based polymers with side chains of perfluorosulfonic acid (the Nation family) are important ion-exchange membrane materials used in practice for electrolysis of NaCl and in certain fuel cells. They show a proton conductivity of 0.01 S cm- at room temperature. However, such fast ionic transport occurs only when they are swollen with water. It is therefore not appropriate to call them solid electrolytes in the tme sense of the word. It was in 1970 that anionic conductivity, though not high, was reported for crown ether complexes such as dibenzo-18-crown-6 KSCN, in which cations are trapped by the ligand. " A few years later much higher cationic (instead of anionic) conduction was found in complexes of a chain-like polyether such as PEO or PPO with alkaline salts here, PEO stands for poly(ethyleneoxide), (CHjCHj-O), and PPO for poly(propyleneoxide)."2>"3 These were the flrst examples of tme polymer solid electrolytes and were followed by a great number of studies. Polymeric electrolytes are advantageous in practice because they are easily processed and formed into flexible Aims. 

The results of ion and water sorption measurements for the two polymers under these solution conditions help to explain this difference. Table 5 " lists the concentrations of various sorbed species and the mole ratio of water to cation/anion in the polymer phase for NaCl and NaOH solution environments. This ratio decreases both in the polymers and in solution with increasing concentration. In solution, the ratio varies from 10.8 to 4.0 over the concentration range of 5-12.5 M NaOH, so that ions in the polymer phase exist in a significantly less aqueous environment compared to the solution phase. As noted by Mauritz and co-workers for perfluorosulfonate membranes, these water contents are insufficient to provide even primary hydration spheres for sodium ions, sorbed anions, and exchange sites, and the likelihood... 

Hie loss of selectivity as a result of membrane dehydration described above is similar to results reported previously for Ag(I)-facilitated transport of alkenes in perfluorosulfonic acid membranes.(13) For cation transport in ion-exchange materials, it is known that loss of membrane hydration results in a dramatic decrease in conductivity suggesting that the cations become much less mobile in dry membranes. Ftesumably, a similar phenomenon is responsible for the diminished benzene fluxes observed for dehydrated PVA-AgX membranes. Even though a Ag(I)-benzene complex can still form, hydration is necessary for the complex to be mobile and thereby provide effective facilitation of benzene. 

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