Nafion micelles

The contact between the metal nanoparticles and Nafion micelles in the CL of a DMFC is affected by carbon-support-particle pore size and distribution [79]. Rao et al. investigated the effect of carbon porosity on the specific activity of the Pt-Ru/C catalyst for methanol oxidation [83]. They found that a higher content of small pores (< 20 nm) containing metal particles, where die Nafion ionomer could not easily enter, resulted in poor contact between die metal nanoparticles and... 

Nafion is a novel and unique family of polymers which consist of a perfluorinated backbone and short pendant chains terminated by sulfonic groups. When swollen in water, the structure of Nafion is believed to resemble that of an inverse micelle (Fig. 3) [16-18], The hydrated SOf head groups are clustered together in a... 

Photolyses of 31-34 in homogeneous solution results in the formation of diphylethanes 39 (5-15%), phenols 38 (5-15%), ortho-hydroxyphenone 36 (40-60%), and para-hydroxyphenones 37 (20-25%). Small amounts of phenyl benzyl ether 35 (3-8%) were also detected. However, photolyses of all of the four esters on NaY zeolite and Nafion only produce ortho rearrangement products 36. Molecular models suggest that esters 31-34 can enter into NaY zeolite internal surface and the inverse micelle of Nafion. We believe that the preference for formation of ort/zo-hydroxyphenones 36 in the products is a consequence of the restriction on diffusional and rotational motion of the geminate radical pair. 

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... 

Figure 19 TEM micrograph of catalyst-Nafion interface showing metal particles supported on carbon agglomerate and Nafion ionomer micelles.

Significantly less cis isomer is formed in the Nafion film. This result in addition to the low level of p formation in the film relative to that in the reversed micelle suggests that the high percentage of cis formed in reversed micelle is due to deprotonation. As expected, the amount of trans- cis isomerization increases with decreasing coverage level. 

Figure 1. Schematic of the reversed micelle model of Nafion...

The ionic groups (-SO3H) form clusters in the chemically stable Teflon-like matrix (-CF2CF2-) [6]. These hydrophilic domains form micelles, surrounded by the hydrophobic fluorocarbon backbone and connected by small channels. The Nafion resin is prepared as a copolymer of tetrafluoroethylene and perfluoro-2-(fluorosul-fonylethoxy)propyl vinyl ether [1,7]. Unlike other resinsulfonic acids, e. g. sulfo-nated polystyrenes (Dowex-50, Amberlyst-15, Amberlite IR-112, etc.), Nafion and related perfluoroalkanesulfonic acids are stable in corrosive environments and the maximum operating temperature is up to 473K [8]. 

Nafion in the bulk phase separates into hydrophobic and hydrophilic SO3H regions. The ionic domains or clusters are inverted micelles surrounded by a fluorocarbon matrix, and the ionic domains are connected by short channels. Because of the inverted micelle within the fluorocarbon matrix, availability of the acid sites for catalysis is greatly diminished. Two approaches were initially used to increase the catalytic activity of bulk Nafion polymer blends (2) and coating a liquid conposition of Nafion on a hydrophobic support (3). 

In an attempt to explain the activity of Nafion it was hypothesized that the SO3H groups were not in the inverted micelle, but rather were exposed at the surface of the micelle. This observation suggested selecting a different hydrophobic material. The next approach was to utilize a liquid composition of Nafion and coat a hydrophobic support such as carbon. The carbon selected was calcined shot coke, a material with very large pores. The mean pore diameter was about lOOOA. 

At low water content the quasi-crystalline motif in the distribution of inverted micelles is strong the size of the micelle is smaller than the persistence length of the bundles of backbone-chains forming the membrane skeleton. A legitimate question arises then how can one build a quasicrystalline structure of inverted micelles (aqueous droplets supported by hydrated sidechains), if the sidechains are attached to the backbones In an attempt to answer this question, a more detailed morphological model of Nafion-type ionomers was suggested [31] a quasi-crystalline arrangement of units cells as depicted in Fig. 1. 

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