Ionic conduction pathways

Ionic-conduction Pathways in Hybrid Membrane Materials... 

The anode layer of polymer electrolyte membrane fuel cells typically includes a catalyst and a binder, often a dispersion of poly(tetraflu-oroethylene) or other hydrophobic polymers, and may also include a filler, e.g., acetylene black carbon. Anode layers may also contain a mixture of a catalyst, ionomer and binder. The presence of a ionomer in the catalyst layer effectively increases the electrochemically active surface area of the catalyst, which requires a ionically conductive pathway to the cathode catalyst to generate electric current (16). 

Figure 8 Video images of the appearance of high-ionic-conductance pathways during iontophoresis of Fe(CN) and Fe. Counterdirectional iontophoresis was conducted across HMS for up to 2 hours at a current density of 40 pA/cm. The dark spots are localized deposits of Prussian blue dye at pore openings. (Reproduced with permission from Ref. 65.)...

The regions of molten ash deposits serve as ionic conduction pathways to complete the electrical circuit. In practical examples of high temperature corrosion under deposits, the mechanism is likely to be far more complex than this simple explanation. 

Nectunis gall bladder, 95% of the ionic conductance pathway follows an extracellular route [205]. 

PVB, tetrabutylammonium perchlorate (TBAP), imidazole (Imz) or PEG2000 were employed as host polymer, salt and additive for the polymer electrolyte. Imz or PEG2000 are the molecules that are effective in forming an ionic conduction pathway above its melting temperature. TBAP also has a character of melting. ... 

Avdeev M, Sale M, Adams S, Prasada Rao R (2012) Screening of the alkali-metal ion containing materials from the Inorganic Crystal Structure Database (ICSD) for high ionic conductivity pathways using the bond valence method. Solid State Ion 225 43-46... 

In polycrystalline materials, ion transport within the grain boundary must also be considered. For oxides with close-packed oxygens, the O-ion almost always diffuses much faster in the boundary region than in the bulk. In general, second phases at grain boundaries are less close packed and provide a pathway for more rapid diffusion of ionic species. Thus the simplified picture of bulk ionic conduction is made more complex by these additional effects. 

The ionic conductivity for various ions in the piP"-alumina structure along the conduction planes shows a maximum for an optimum size of the ions. It should be neither too small nor too big to fit the available pathways in the lattice [8]. 

The electrocrystallization and characterization of a novel molecular metal which displays both electronic and ionic conduction has been reported. The complex Li0.6(15-crown-5-ether)[Ni-(dmit)2] H20 is composed of stacks of [Ni(dmit)2] units which provide pathways for electronic conduction. The stacks are separated by parallel stacks of 15-crown-5-ether moieties in a channellike formation which facilitates ion conduction. The salt has a room temperature conductivity of 240 Scm-1. Temperature-dependent magnetic susceptibility and NMR measurements were used to prove the existence of Li+ movement within the crown ether channels.1030... 

Successive H-bond urea self-assembly of 4 and sol-gel transcription steps yield preferential conduction pathways within the hybrid membrane materials. Crystallographic, microscopic and transport data confirm the formation of self-organized molecular channels transcribed in solid dense thin-layer membranes. The ionic transport across the organized domains illustrates the power of the supramolecular approach for the design of continual hydrophilic transport devices in hybrid membrane materials by self-organization (Figure 10.8) [42-44]. 

The activation energy represents the ease of ion hopping, as already indicated above and shown in Fig. 2.5. It is related directly to the crystal structure and in particular, to the openness of the conduction pathways. Most ionic solids have densely packed crystal structures with narrow bottlenecks and without obvious well-defined conduction pathways. Consequently, the activation energies for ion hopping are large, usually 1 eV ( 96 kJ mole ) or greater and conductivity values are low. In solid electrolytes, by contrast, open conduction pathways exist and activation energies may be much lower, as low as 0.03 eV in Agl, 0.15 eV in /S-alumina and 0.90 eV in yttria-stabilised zirconia. 

The mechanism of action of inert barrier pigments is commonly stated to be to Increase the diffusion pathway to the substrate it is also possible that pigments may tend to block or prevent the formation of pathways for direct ionic conduction to the substrate. 

This generalized structure was mimicked by a very simplified artificial molecule 1. The hydrophilic core part 2 was substituted simply by an oligoether carboxylate anion. The carboxylate may act as the polar ionic head group outside the membrane and the ether part of the molecule may be located in the interior part of the membrane to make an ion-conducting pathway. The molecular lengths were adjusted to fit the lipid monolayer in an extended or a helical conformation, with n being 2 or 3 in 1. The hydrophobic exterior was substituted by dioctadecyldimethyl-ammonium cation, which was ion-paired with the carboxylate. 

Frizzell RA, Schultz SG. Ionic conductances of extracellular shunt pathway in rabbit ileum. Influence of shunt on transmural sodium transport and electrical potential differences. J Gen Physiol 1972 59 318-346. 

One largely investigated system in the field of silver ion conductivity is the Rbl-AgI system, including the fast ionic conductor, RbAgJj [19,28,29], Structural investigations of these compounds started 40 years ago and the field has been active during the last four decades. The mechanism of conduction and the favored conduction pathway in ionic conductors is important for the development of solid-state batteries [28],... 

---