Membrane with Nafion cation-exchange

For laboratory-scale reactions, this electrocatalytic AD generally is performed in a glass H-type cell in which the anode and cathode compartments are separated by a semipermeable Nafion cation-exchange membrane and platinum electrodes are used. A 5% aqueous solution of phosphoric acid is used in the cathode compartment, and the reaction in the anode compartment is stirred vigorously, Under a controlled anode potential of 0.4 V (vs. Ag/AgCl) and with (DHQD)2-PHAL as chiral ligand, a-methylstyrene was converted to 7 -2-pheny 1-1,2-propanediol in 15 h with the electrical consumption of 2.1 F/mol. The product was isolated in 100% yield with 92% ee [ 37],... 

In the zinc/bromine cell, the halogen is stored in the electrolyte as a polybromide (mainly Bra ) and hence a separator is essential Nafion cation exchange membrane has again been used. The electrolyte is aqueous zinc bromide with tetraalkyl-ammonium bromide to aid the bromine complex formation. 

The upper part of Fig. 2 shows the molecular structure of the perfluorinated Nafion cation exchange membranes with sulfonic acid as well as with carboxylic acid groups as fixed ions. These are covalently bonded at the end of side chains of the PTFE (polytetrafiuoroethylene) polymer backbone. The polymer has excellent chemical and thermal stability, similar to PTFE [9]. 

As mentioned, this review is focused primarily on a survey of the vast literature dealing with the structure and properties of Nafion in the sulfonic acid and cation exchanged sulfonate forms. The literature on the carboxylate version is sparse and currently of lesser interest, as its application seems to be limited to membranes in chlor-alkali cells, and since it is a... 

For PEMFCs, the solid electrolytes are polymer membranes polymers modified to include ions, usually sulfonic groups. One of the most widely used membranes today is the polymer Nafion , created by the DuPont company. These membranes have aliphatic perfluorinated backbones with ether-linked side chains ending in sulfonate cation exchange groups [6, 7], Nafion is a copolymer of tetrafluoroethylene and sulfonyl fluoride vinyl ether [8] and has a semi-crystalline structure [9], This structure (which resembles Teflon ) gives Nafion long-term stability in oxidative or reductive conditions. The sulfonic groups of the polymers facilitate the transport of protons. The polymers consist of hydrophilic and hydrophobic domains that allow the transport of protons from the anode to the cathode [10, 11],... 

Fig. 9 Solid polymer electrolyte (SPE) cell with cation exchanger membrane Nafion [13].

Notes. (1) ith Na2SO4 in the strip solution instead otH SO, Only Nafion-120 membrane. (2) Initial composition feed aqueous equimolar solutions of Zn(ll), Cu(ll), Mn(II), Ni(ll), and Co(II) sulfates, 2 x 10 mol/dm each carrier 0,1 mol/dm FTP, or PGF or PAGF aqueous solution strip 0,1 mol/dm aqueous solution of sulfuric acid. Membrane barriers cation-exchange Naflon-120. (3) Soutee From Ref [90] with permission. 

Perfluorinated ionomers such as Nafion are of significant commercial importance as cation exchange membranes in brine electrolysis cells ( 1). Outstanding chemical and thermal stability make this class of polymers uniquely suited for use in such harsh oxidizing environments. The Nafion polymer consists of a perfluorinated backbone and perfluoroalkylether sidechains which are terminated with sulfonic acid and/or carboxylic acid functionality. 

General Considerations. In order to study cation exchange equilibria it is necessary to determine the rate at which exchange equilibrium is attained. Exchange rates are relatively rapid for 1200 EW Nafion. A IT -form membrane, when immersed in aqueous NaCl solution, attains 90% conversion to the Na" "-form in less than two minutes (5). This time interval increases to 40 min for conversion to the Cs" "-form. This increase in equilibration time is attributable to the anomalously low diffusion coefficient of Cs" " in the polymer phase (6). Even in this case equilibration times of a few hours are sufficient to ensure complete reaction. Another factor to consider is whether all sulfonate sites are available for exchange with various cations. 

A recent study (12) has shown that Nafion is also suitable for use in water electrolyzers with alkaline solution as the supporting electrolyte. The major charge carrier is the alkali metal ion because of the negligible IT " ion concentration in alkaline solution and the Off ion rejection capability of the cation exchange membrane. The current efficiency of the cell is related to the inhibition of the transport of gaseous products across the separator. Thus, the ionic groups of the membrane are not important, in this case, because alkaline solution is the major electrolyte, and the migration of any ionic species across the membrane would not affect the current efficiency of the cell (33). 

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