Sodium Nafion

Spectra of water in the sodiun salt of Nafion were studied in considerable detail in ref. 4, with special attention to the spectral profiles of the stretching fundamentals of isotopically isolated HDO. Isotopic isolation is achieved experimentally by observing the OD stretching fundamental of a sample with D/H ratio of 1/10 or less, or alternatively the OH stretching fundamental of a sample with H/D ratio of 1/10 or less. These bands are much simpler and thus easier to interpret structurally than the stretching fundamentals of either H20 or D20. The spectra described in ref. 4 have been fully confirmed by further work in the author s laboratory, except that the samples described as "fully hydrated" have now been found to correspond to water content below 3 H20/-S03 whereas full hydration for sodium Nafion corresponds to some 14 H20/-S0o . With this re-interpretation, the results of ref. 4 will be summarized in the next two sections. 

Saponification to the sulphonic acid yields the product marketed as Nafion. This material is said to be permselective in that it passes cations but not anions. It is used as a membrane material in electrochemical processes, in for example the manufacture of sodium hypochlorite. 

Later, Du Pont in America developed its own ionically conducting membrane, mainly for large-scale electrolysis of sodium chloride to manufacture chlorine, Nafion , (the US Navy also used it on board submarines to generate oxygen by electrolysis of water), while Dow Chemical, also in America, developed its own even more efficient version in the 1980s, while another version will be described below in connection with fuel cells. Meanwhile, Fenton et al. (1973) discovered the first of a... 

Today, the term solid electrolyte or fast ionic conductor or, sometimes, superionic conductor is used to describe solid materials whose conductivity is wholly due to ionic displacement. Mixed conductors exhibit both ionic and electronic conductivity. Solid electrolytes range from hard, refractory materials, such as 8 mol% Y2C>3-stabilized Zr02(YSZ) or sodium fT-AbCb (NaAluOn), to soft proton-exchange polymeric membranes such as Du Pont s Nafion and include compounds that are stoichiometric (Agl), non-stoichiometric (sodium J3"-A12C>3) or doped (YSZ). The preparation, properties, and some applications of solid electrolytes have been discussed in a number of books2 5 and reviews.6,7 The main commercial application of solid electrolytes is in gas sensors.8,9 Another emerging application is in solid oxide fuel cells.4,5,1, n... 

In the membrane-cell process, highly selective ion-exchange membranes of Du Font s Nation type are used which allow only the sodium ions to pass. Thus, in the anode compartment an alkali solution of high purity is produced. The introduction of Nafion-type membranes in chlor-alkali electrolyzers led to a significant improvement in their efficiency. Today, most new chlor-alkafi installations use the membrane technology. Unfortunately, the cost of Nafion-type membranes is still very high. 

A fiber optic sensor for the determination of sodium was reported by Burgess.<52) A bifurcated fiber with a reference fiber 5 mm apart from the tip was used to observe the changes of bromothymol blue (Amax = 620 nm) attached to Nafion in the presence of sodium ions. As the tip was saturated, the probe was renewed with fresh reagent. However, the epoxy holding the fibers was prone to damage from high sodium concentrations of around 2.5 M and the sensitivity of analysis was low. 

The relative viscosity rj and storage modulus G were determined by Cirkel and Okada in experiments using a rheometer in oscillatory rotational mode and Couette sample geometry as a function of Nafion volume fraction, cp, and angular frequency, a>, for the acid and sodium forms at 25 °C. Parallel experiments... 

The rheological properties change behavior, relative to more dilute solutions, above cp = 0.2, where non-Newtonian behavior is then exhibited. The power law dependence of rj on cp is in harmony with the Zimm rather than the Rouse model, which suggests that hydrodynamic interactions between these polymers, in a mean field sense, are important. Electrical properties also begin to deviate for Nafion solutions above cp = 0.2, and mechanical percolation is essentially the same for the sodium and acid forms. 

Nafion is a copolymer of poly(tetrafluoroethylene) and polysulfonyl fluoride vinyl ether. It has fixed anions, which are sulfonic acid sites, and consequently, by electroneutrality, the concentration of positive ions is fixed. Furthermore, the transference number of protons in this system is 1, which greatly simplifies the governing transport equations, as seen below. There can be different forms of Nafion in terms of the positive counterion (e.g., proton, sodium, etc.). Most models deal only with the proton or acid form of Nafion, which is the most common form used in polymer-electrolyte fuel cells due to its high proton conductivity. 

The surface of a carbon electrode was at first coated with a thin film of an anionic polymer such as sodium poly(styrene-sulfonate) 95) or nafion 96) (thickness thousand A) then the cationic Ru(bpy)2+ was adsorbed in the anionic layer electrostatically. The modification was also made by coating water insoluble polymer pendant Ru(bpy)2 + ( ) from its DMF solution 97). These Ru(bpy) +/polymer modified electrode gave a photoresponse in the MV2+ solution with the Pt counter electrode 95-97) The time-current behaviours induced by irradiation and cutoff of the light under argon are shown in Fig. 28. It is interesting to see that the direction of the photocurrent reversed at the electrode potential of ca. 0.4 V (vs. Ag—AgCl) under... 

More recently, Tsuchida et al. [107] achieved a Na+ ionic conductivity of 1 x 10-6 fi-1 cm-1 in a composite film made from Nafion (perfluorosulfonate ions) and diendo-acetylated polyoxy ethylene, due apparently to enhanced dissociation of the sodium perfluoro sulfonated groups in the composite matrix. 

The fundamental principle of SPE reactors is the coupling of the transport of electrical charges, i.e. an electrical current with a transport of ions (cations or anions), through a SPE membrane due to an externally applied (e.g. electrolysis) or internally generated (e.g. fuel cells) electrical potential gradient. For example, in a chlorine/alkaline SPE reactor (Fig. 13.3), the anode and cathode were separated by a cation-SPE membrane (e.g. Nafion 117) forming two compartments, containing the anolyte (e.g. 25 wt% NaCl solution) and the catholyte (e.g. dilute sodium hydroxide), respectively. 

The main disadvantage of Nafion is that the inverse micelles in the structure must contain water if the polymer is to conduct protons or sodium ions, and dehydration of the membrane at higher temperatures leads to failure. A second disadvantage, particularly serious in... 

Olefins Silver tetrafluoroborate Silver-I, sodium-I Composite (poly ethylene oxide) Nafion-poly(pyrrole) Pinnau and Toy (2001) Sungpet et al. (2001)... 

Regenesys uses DuPont s Nafion (Section 6.1.7) as the perm-selective sodium ion transfer membrane, separating the two half cells. Figure 2.1. Diffusion of sodium ions in the concentration difference across the Nafion membrane is one of the irreversibilities of the system. The low-cost plastic (e.g. polyethylene) tanks and pipework are treated with fluorine to provide bromine resistance, and are able to operate with, and contain, both electrolytes at ambient temperature. 

The ionically bonded sodium bromide receives two electrons, and loses two +ve sodium cations to form sodium tri-bromide (see Porterfield, 1993). An alternative homogeneous reaction route to tri-bromide is to bubble bromine (Br2) through aqueous sodium bromide. Accordingly, the electrochemically formed tri-bromide has an associated atmosphere of bromine from homogeneous dissociation. Such bromine is corrosive, but is contained by the fluorinated plastic tanks and by one side of the (Na ) Nafion perm-selective membrane. Nafion is itself a compound with many fluorine atoms. Bromine is very much lower than fluorine in chemical reactivity, so that the bromine containment strategy is complete, and the selection of the bromide electrolyte seen to be a well-considered choice. 

Commercially available 85% MCPBA is generally employed in chlorinated hydrocarbon solvents at room temperature. Reaction times are typically a few hours to several days. Buffers utilized include disodium hydrogen phosphate, sodium acetate and sodium bicarbonate, the catalytic effect of which has been occasionally noted. Acid catalysis with sulfuric acid or Nafion-H are alternatives. Oxidations have been performed at elevated temperature with the aid of radical scavengers. "... 

Bromine azide, prepared by different methods, undergoes addition to alkenes stereoselectively by an ionic mechanism 33- 37 (Table 2). Improved yields are obtained by using azidotrimethyl-silane and A-bromosuccinimide (NBS) in the presence of Nafion-FI as catalyst33. Alternatively, A-bromosuccinimide may be added to the mixture of the alkene and sodium azide in dimeth-oxyethane/water34. Aziridines were prepared stereoselectively by reduction of (i-bromo azides with lithium aluminum hydride34. 

Recently, imidazole has been successfully applied as a catalyst for the acetylation of carbohydrates in acetonitrile [199]. A variety of other catalysts in combination with excess of acetic anhydride and solvent includes sodium acetate [200], sulfuric acid [201], perchloric acid [202], and a number of Lewis acid catalysts such as, iodine [203], Sc(OTf)3 [204], Cu(OTf)2 [205], C0CI2 [206], BiOCl-SOCl2 [207], LiC104 [208], FeCls [209], BiCls [210], and a series of heterogeneous catalysts such as, montmorillonite K-10 [211], zeolites [212], nafion-H [213], HC104-Si02 [214], or molecular sieves [215]. Recently, a ZnCl2-sodium acetate combina-... 

Palladium electroless deposition was used for coating of Nafion 117 for the application as a membrane in direct methanol fuel cell.59 After the activation of the polymer membrane (Nafion 117) with Pd(II) complexes, reduction was carried out with sodium boro-hydride, NaBH4. Further, autocatalytic deposition of palladium was performed using a commercially available solution. Compared to bare Nafion, the Nafion/Pd composites considerably reduced methanol crossover. This resulted in enhanced cell performance, which was attributed to the existence of the Pd layer at the surface of polymer. 

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