Inhibition electrolyte-layer

In planar SOFCs, individual cathode, anode, and electrolyte layers have been deposited by PS [109-111], as well as coatings on interconnect materials and full cells [108, 110, 112]. In addition to the interconnect layers themselves in tubular SOFCs, dense protective layers with good adhesion have also been deposited to protect planar SOFC interconnects from oxidation [110], and diffusion barriers to inhibit inter-diffusion between the interconnects and anodes have been produced by PS [113]. 

EQCM study demonstrated that PEG and Cl stabilize the formation of Cu+ at potentials near open circuit [248]. Accordingly, it was concluded that neither Cu+ nor Cu2+ from the electrolyte are necessarily required to form the inhibiting surface layer at more negative potentials [248]. Thus, some debate remains as to the importance of Cu+ to the formation and longevity of the PEG—Cl blocking layer [241, 248-250]. However, if Cu+ is not required, the physical basis by which non-ionic PEG interacts with the halide covered electrode needs to be rationalized. 

Thus, the transport of hydrated ions and chemical debonding processes can be studied by means of the SKP. Fig. 31.6 shows the potential distribution measured with the SKP when a thin electrolyte layer enters the interface between an adhesive and an iron surface covered by a thin (about 6 nm) nonconducting SiOx layer precipitated by a plasma-polymerization process [51, 52]. The SiO layer inhibits the electron-transfer reaction. Consequently, no corrosive degradation of the interface takes place (see Section 31.3.2.1). However, as the adhesion of the epoxy adhesive to the siUca-Uke layer is weak, the polymer is replaced by... 

The starting point for such classification is the point of interference with the above sketched corrosion mechanism either in a phenomenological or in a mechanistic way, A simple system for classification, which will be discussed in more detail later, is based on whether the inhibitor interferes with the anodic or cathodic reaction. Thus inhibitors are classified as anodic or cathodic inhibitors. However, this distinction was shown to be too simplistic and a more complex classification was worked out by H. Fischer (JJ on the basis of where, instead of how, in the complex interphase of a metal-electrolyte system the inhibitor interferes with the corrosion reactions. The metal-electrolyte interphase can be visualized as consisting of (a) the interface per se, and (b) an electrolyte layer interposed between the Interface and the bulk of the electrolyte. On this basis Fisher distinguished as shown in Table 1, between "Interface Inhibition" and "Electrolyte Layer Inhibition."... 

Causes of Interface-Inhibition and Electrolyte-Layer-Inhibition... 

All of the above inhibition phenomena are visualized as taking place immediately on the metal surface of the corroding specimen. Apparently, there are ways that one can affect corrosion reactions by interfering with processes which are not on the surface, but those in the vicinity of the surface, namely in the electrolyte layer closest to the surface. Electrolyte layer inhibition may hinder the following partial steps of electrode reactions ... 

This is called the electrochemical electrolyte layer inhibition. 

Metals exposed to humid atmosphere corrode by an electrochemical mechanism due to the formation of a thin electrolyte layer on the metal surface (Chapter 3.1, this volume). This type of corrosion can be controlled by Vapor-phase Corrosion Inhibitors (VCIs), that is, volatile inhibiting substances that allow vapor-phase transport to the corroding surface (examples are amines, benzoates, imidazoles, or triazoles [3]). The vapor pressure should be sufficiently high to ensure a protective surface concentration of the inhibitor, but low enough to prevent premature depletion of... 

In the case of nitrobenzoates, for example, it has been claimed that an acceleration of the cathodic partial process by reduction of the nitro group may lead, in addition to the effect of oxygen in the thin electrolyte layer, to a complete passivation of iron or ordinary steels. Contributions from the two parts of the dissociated molecule to the inhibitive effect are very likely. 

Foreign cations can increasingly lower the yield in the order Fe, Co " < Ca " < Mn < Pb " [22]. This is possibly due to the formation of oxide layers at the anode [42], Alkali and alkaline earth metal ions, alkylammonium ions and also zinc or nickel cations do not effect the Kolbe reaction [40] and are therefore the counterions of choice in preparative applications. Methanol is the best suited solvent for Kolbe electrolysis [7, 43]. Its oxidation is extensively inhibited by the formation of the carboxylate layer. The following electrolytes with methanol as solvent have been used MeOH-sodium carboxylate [44], MeOH—MeONa [45, 46], MeOH—NaOH [47], MeOH—EtsN-pyridine [48]. The yield of the Kolbe dimer decreases in media that contain more than 4% water. 

So far, several examples have been given of the inhibition of electrocatalytic processes. This retardation is a result of occupation of the catalyti-cally more active sites by electroinactive components of the electrolyte, preventing interaction of the electroactive substances with these sites. The electrode process can also be inhibited by the formation of oxide layers on the surface and by the adsorption of less active intermediates and also of the products of the electrode process. 

Very interesting behavior of incorporating anions can be observed when a multicomponent electrolyte is used for oxide formation. Here, anion antagonism or synergism can be observed, depending on the types of anions used. The antagonism of hydroxyl ions and acid anions has been observed in a number of cases. Konno et a/.181 have observed, in experiments on anodic alumina deterioration and hydration, that small amounts of phosphates and chromates inhibit oxide hydration by forming monolayer or two-layer films of adsorbed anions at the oxide surface. Abd-Rabbo et al.162 have observed preferential incorporation of phosphate anions from a mixture of phosphates and chromates. 

Addition of sodium dodecyl benzene sulfonate to dilute alkaline electrolyte depresses the passivation of zinc surface [275]. Owing to the dodecyl benzene sulfonate adsorption, the passive layer on zinc has a loose and porous structure. Zinc electrodissolution was inhibited by the presence of sodium metasdicate [276] and some acridines [277]. The protection effect was described by a two-parameter equation. 

In discussing Reaction (F), we remarked that other anions are observed to compete with OH " in the Stern layer. This sort of electrolyte inhibition is widely observed, and the dependence of the inhibition on both the size and charge of the ions generally corresponds to expectations. For example, in the base-catalyzed hydrolysis of carboxylic esters in the cationic micelles, anions inhibit the reaction in the order N03 > Br " > Cl > F. For acid-catalyzed ester... 

Water content affects many processes within a fuel cell and must be properly managed. Proton conductivity within the polymer electrolyte typically decreases dramatically with decreasing water content (especially for perfhiorinated membranes such as Nation ), while excessive liquid water in the catalyst layers (CLs) and gas diffusion layers (GDLs) results in flooding, which inhibits reactant access to the catalyst sites. Water management is complicated by several types of water transport, such as production of water from the cathode reaction, evaporation, and condensation at each electrode, osmotic drag of water molecules from anode to cathode by... 

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