Polyelectrolyte Nafion

T.P. Henning and A.J. Bard, Polymer films on electrodes. XI. Electrochemical behavior of polymer electrodes produced by incorporation of tetrathiafulvalenium in a polyelectrolyte (Nafion) matrix, J. Am. Chem. Soc., 1983, 105, 613-621. 

Polyelectrolyte - Nafion Membranes for Direct Methanol Fuel Cells. Adv. Mater. Vol.18, pp. 1068-1072. 

Composites of PPV in films of the polyelectrolyte Nafion have been synthesized by electrostatically binding the dicationic monomer to the film, followed by treatment with base and thermal conversion (48). Ordered nanocomposites of PPV have been synthesized from mixtures of polymerizable lyotropic liquid crystals with PPV precursors (49). Photopolymerization of the host results in a hexagonal architecture, which can be fabricated into fibers and thin-films. A significant enhancement in photoluminescence of the composite relative to PPV was found. 

S.P. Jiang, Z. Liu, Q. Tian, Layer-by-layer self-assembly of composite polyelectrolyte-nafion membranes for direct methanol fuel cells, Adv. Mater. 18 (2006) 1068-1072. 

In polyelectrolyte gels the variation of pH or salt concentration (cs) causes a swelling or shrinkage. Therefore, in this case chemical energy is transformed to mechanical work (artificial muscles). An increase of cs (or a decrease of temperature) makes the gel shrink. Usually, the shrinking process occurs smoothly, but under certain conditions a tiny addition of salt leads to the collapse of the gel [iii, iv]. Hydration of macroions also plays an important role, e.g., in the case of proton-conductive polymers, such as -> Nafion, which are applied in -rfuel cells, -> chlor-alkali electrolysis, effluent treatment, etc. [v]. Polyelectrolytes have to be distinguished from the solid polymer electrolytes [vi] (- polymer electrolytes) inasmuch as the latter usually contain an undissociable polymer and dissolved small electrolytes. 

In the presence of water this polyelectrolyte is an excellent protonic conductor, although it suffers some degradation in a working fuel cell. Probably the most successful polyelectrolytes developed so far are based on perfluorinated polymers, the first of which was Nafion , with the structure ... 

The upper temperature at which polyelectrolytes can be utilised can be extended by substitution of a polar organic solvent for water in the electrolyte. Conductivities of up to 1 Q 1m 1 have been obtained for sulphonated PEEK using anhydrous pyrazole and imidazole at 200 C (Kreuer et al., 1998). Solvents used to form electrolytes in Nafion membranes have included alcohols, amines, propylene carbonate, dimethoxyethane, dimethyl form-amide, dimethyl sulphoxide and TV-methyl pyrrolidone. Typical conductivities for the latter three solvents are 10 to 1 O-lm 1 at room temperature, and the conductivity is thermally activated with activation energies in the range 9 to 24 kJ/mol (Doyle et al., 2001). 

Most of the ion-exchange resins consist of polyions. A typical one is also the most well known Nafion, the structure of which is shown in Fig. 2.77 the figure also shows the structure of some proteins (these are often polyelectrolytes). 

The important acid activity in Nafion is appropriately represented by trifluo-romethane sulfonic (triflic) acid, CF3SO3H see Fig. 1.5. Dielectric spectroscopy has suggested that a significant amount of triflic acid is not dissociated in the ionic melt at 50% mole fraction of water (Barthel et al, 1998). But the deprotonation chemistry of hydrated triflic acid hasn t been experimentally studied over the wide range of hydration and temperature that would be relevant to the function of sulfonate-based polyelectrolyte membrane materials. 

The methoxylation of A-ethyl carboxamides has been studied especially intensively, as the produced A-(a-methoxyethyl)amides may be transformed to the technically interesting vinyl amides by methanol elimination [209]. Glassy carbon anodes together with graphite or steel cathodes and tetramethylammonium methyl sulfate as conducting salt proved to be the best conditions. Alternatively, the SPE cell using Nafion as polyelectrolyte is practicable [218,219]. 

Considerable progress has been made in the solution theory of poly-electrolytes. However, for the condensed-phase analogs of polyelectrolytes, ionomers, this is not the case. Eisenberg (1) has put forth an initial theory of ionomer structure that contains conceptual formalisms of general use. His theory has been consulted extensively in the work reported here. Ponomarev and Ionova (2) have attempted to construct a sophisticated statistical mechanical model to describe the thermodynamics of ionomers. Recently, Gierke (3) has described a theory of ion transport in the Nafion ionomer based on a specific molecular organization. 

There are two types of polymeric electrolyte, based on their conduction mechanisms. The first group is the polyelectrolyte in which the polymer itself contains an anionic or cationic group, usually on a side chain. The counter-ions for these groups are typically small, inorganic ions that are mobile within the polymer matrix. Nafion, a perfluorinated sulfonated ionomer made by du Pont, is an example of this type of electrolyte [1]. Nafion has been used as the electrolyte in several amperometric gas sensors. 

Studies on various electrodes functionalized by polyelectrolytes have been reported chemiluminescene of Nafion-coated membranes,334 behavior of violo-gen-based chemically modified electrodes,335 and polymer electrodes with incorporated tetrathiafulvalenium in a Nafion membrane.336... 

The electrodeposition method is particularly useful for preparation of poly pyrroles since a wide range of materials can be prepared. For example in our laboratories a range of polymers containing sulfonated aromatics (Table 1) have been prepared as have composite polymers based on incorporation of polyelectrolytes such as polyvinylsulfonate or Nafion (4). This flexibility in synthesis is extremely important since the polymer composition determines the transport properties that can be achieved. 

We have also prepared stand alone membranes containing polyelectrolytes such as PVS or Nafion (11). Again the transport characterstics are affected and with these materials only cation can be transported. 

Figure 13.3 Visualisation of the water-filled channels in polyelectrolyte model for Nafion, as simulated by Elliott and Hanna [41]. The atomic fragments have been deleted for clarity, replaced by three-dimensional Connolly surfaces produced from models containing (a) 6.3% and (b) 15.9% water by mass. In the original paper, the surfaces were coloured by proximity to polar and non-polar regions.

Special attention deserves Nafion layered membrane prepared by the LBL self assembly of polyelectrolytes [25, 167-171]. A high selectivity membrane was prepared by Tang et al. [25] by self-assembling multi-layer Pd nanoparticles onto Nafion, using poly(diallyl dimethylammonium chloride) (PDAC) for charging the Pd particles. A Nafion 112 membrane was immersed in a Pd/PDDA dispersion and then in a Nafion dispersion. The process was repeated five times to obtain a multilayer self-assembly Nafion composite that shows a small decrease in conductivity (from 112 mS.cm for Nafion 112 to 81 mS.cm for the composite). However, the reported methanol permeability was reduced by a factor 0.0085 (out of scale in Fig. 6.22), leading to k 85. This composite membrane, whose strucmre is depicted in Fig. 6.24, was not tested in a DMFC. 

Hasani-Sadrabadi MM, Dashtimoghadam E, Majedi FS, Kabiri K (2009) Nafion/bio-functionalized montmorillonite nanohybrids as novel polyelectrolytes membranes for direct methanol fuel cells. J Power Sources 190 318-321... 

Kim SH, Song K (2011) Preparation and characterization of Nafion/sPOSS polyelectrolyte nanocomposite membranes for direct methanol fuel cell applications. J Ind Eng Chem 17 170-173... 

Cui ZM, Li NW, Zhou XC, Liu CP, Liao JH, Zhang SB, Xing W (2007) Stuface modified Nafion membrane by casting proton-conducting polyelectrolyte complexes for direct methanol fuel cells. J Power Sources 173 162-165... 

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