Electrolytes, aqueous electronic conductance

Composite structures that consist of carbon particles and a polymer or plastic material are useful for bipolar separators or electrode substrates in aqueous batteries. These structures must be impermeable to the electrolyte and electrochemical reactants or products. Furthermore, they must have acceptable electronic conductivity and mechanical properties. The physicochemical properties of carbon blacks, which are commonly used, have a major effect on the desirable properties of the conductive composite structures. Physicochemical properties such as the surface... 

This study, in conjunction with that discussed in 12.2.1.2, show that when using aqueous electrolytes or Nafion saturated with H20, the induction of NEMCA on finely dispersed noble metal catalysts is rather straightforward. The role of the electronically conducting porous C support is only to conduct electrons and to support the finely dispersed catalyst. The promoting species can reach the active catalyst via the electrolyte or via the aqueous film without having to migrate on the surface of the support, as is the case when using ceramic solid electrolytes. 

In formamide electrolyte containing fluoride ion, the starting anodization current does not drop instantly as observed in aqueous bath. The gas evolution which is indicative of electronic conduction was observed at the anode. The anodization current drops steeply thereafter due to the initial formation of an insulating oxide layer, see Fig. 5.10. In this region, electronic conduction decreases due to the blocking action of the formed oxide, and ionic conduction increases. Once the oxide layer is completely formed over the entire exposed surface of the anode, electronic conduction becomes negligible and ionic conduction dominates the mechanistic behavior. Nanotube formation reduces the surface area available for anodization with a correlated decrease in current density, while deepening of the pore occurs. 

The specific conductivity (y) is a measure of the mobility of ions in an electrolyte or electrons in a metallic conductor. Thus, y is about 1 or 107 S/m for a 0.1 kmol/m3 aqueous salt solution or for a metal such as iron, respectively. Such a difference in charge mobility makes the temperature dependence of % [i-e.,(l/x)3x/97k] positive for ions of about 2.5% per K, but negative for metals and alloys of approximately an order of magnitude lower (Prentice, 1991). 

Valve metals — Metals that form a compact, electronic insulating passive layer when anodized in aqueous electrolyte, exhibiting asymmetric conductivity blocking anodic reactions, except at very high voltages. Valve metals include aluminum, - titanium, tantalum, zirconium, hafnium, and niobium. Some other metals, such as tin, may exhibit valve-metal properties under specific conditions. 

Multifarious patterns of differently functionalized alkanethiol SAMs have been mapped to single-molecule and sub-molecular resolution by in situ STM in aqueous electrolyte, strongly supported by electrochemical studies of reductive desorption in particular. In situ STM is, however, rooted in electronic conductivity and quantum mechanical tunneling. Theoretical support is therefore needed in detailed image interpretation of all the many facets of alkanethiol-based SAM packing and in situ STM contrasts ]163]. 

Metal sulfides and several important oxides display n-type or p-type semiconducting or nieialiic properties. As a result of their electronic conductivity, cermin minerals can participate in coupled charge tmasfer processes aanlogous to a metal corroding in an electrolye, and the kinetics of leaching can be related 1o the potential of the solid in contact with the aqueous electrolyte. 

The NEMCA effect does not appear to be limited to any specific type of catalytic reaction, metal catalyst or electrolyte, particularly in view of the recent demonstration of NEMCA using aqueous electrolytes. The catalyst, however, must be electronically conductive and the only report of NEMCA on an oxide catalyst is for the case of Ir02 which is a metallic oxide. It remains to be seen if NEMCA can be induced on semiconductor catalysts. 

Almost any metal electrode may be applied. One must take care not to operate too close to the limits of the electrochemical window of the used combination of electrode and electrolyte solution in order to avoid catalytic or undesired side effects such as gas evolution reactions. When using thin film electrodes, such as in combined quartz crystal microbalance (QCMB) studies, extreme care must be taken not to scratch the metal surface, usually gold, in order to maintain an electronically conductive path within the electrode. Examples of such experiments are given in the literature [4-7]. Also, when using metal electrodes in aqueous solutions, the background voltammogram should be examined very closely as the formation of surface oxides or the like may be mistaken for signals of the solid under study. 

In spite of these analogies the electrochemistry of solids is more complex than the electrochemistry in aqueous solutions. So it must be noted that apart from ionic conduction, solids often show an electronic conductivity, caused by electrons or electron defects, which may be predominant in many cases over the ionic conduction. In good solid electrolytes the conduction of the electrical current is caused exclusively by the ions—in most cases practically by only one kind of ion present in a crystal. 

It is interesting to address the electronic property of CH3 Si(lll)-(l x 1) not only for the purpose of application as the lithography resist layer hut also as the basis of nanoelectronic devices. The I-V characteristic curve of STM reached a peak at —1.5 eV in the occupied levels additionally to the I-V curve of H Si(lll). This peak corresponds to the electronic state that contributes to electronic conduction through the surface. This conducting property is also materialized in an electrolytic phenomenon. Niwa etal. [61] demonstrated that CH3 Si(lll)-(1x1) as an electrode in an aqueous... 

Electrolytes are distinguished from pure electronic conductors by the fact that the passage of an electric current is only insured by displacement of charged species called ions and hence accompanied by a transfer of matter. Therefore, electrolytes are entirely ionic electrical conductors without exhibiting any electronic conductivity (i.e., no free electrons). They can be found in the solid state (e.g., fluorite, beta-aluminas, yttria-stabilized zirconia, and silver iodide), liquid state (e.g., aqueous solutions, organic solvents, molten salts and ionic liquids), and gaseous state (e.g., ionized gases and plasmas). The ions (i.e., anions or cations)... 

The Ti02 film, being an n-type semiconductor, is electronically conductive. As a cathode, titanium permits electrochemical reduction of ions in an aqueous electrolyte. On the other hand, very high resistance to anodic current flow through the passive oxide film (i.e., significant anodic polarization) can be expected in most aqueous solutions. Elevated anodic pitting (breakdown and repassivation) potentials can also be expected with many titanium alloys. 

In aqueous acidic electrolytes the ionic conduction is provided by an H ion, respectively H (H20)n . As mentioned above, H" is created by the anodic oxidation of H2 as a fuel, and the conducting species is transported to the cathode, where it is consumed in the direct (ideally four electrons) cathodic reduction of molecular oxygen. Hereby, water as the reaction product appears at the cathode side of the cell. 

The redox electrochemistry of a Fe /Fe couple is easily accomplished on a phthalocyanine-coated electrode with peak separations comparable to that of platinum [141,142,163,164]. Phthalocyanine-coated electrodes are found to be efficient electrocatalysts to catalys catechol, p-benzoquinone and oxalic acid oxidations [120,150]. The electrochemical activity of these electrodes may be due to the high voltage, surface area, high electronic conductivity and redox behaviour of phthalocyanine, vanadium phthalocyanine and other phthalocyanines have been prepared by vapour deposition and show photoelectrochemical responses when dipped in aqueous electrolytes [244-249]. Polymeric phthalocyanines of Co and Fe are coated on active carbon and are shown to give catalytic properties for dioxygen reduction and thiol oxidations. Dioxygen chemisorption and ammonia absorption of metallo... 

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