Surface electronic insulation

Another influence that electrolyte materials have on the cycle life of a practical lithium cell results from the evolution of gas as a result of solvent reduction by lithium. For example, EC and PC give rise to [53] evolution of ethylene and propylene gas, respectively. In a practical sealed-structure cell, the existence of gas causes irregular lithium deposition. This is because the gas acts as an electronic insulator and lithium is not deposited on an anode surface where gas has been absorbed. As a result, the lithium cycling efficiency is reduced and shunting occurs. 

Direct-current sputtering is not generally applicable for the preparation of thin-film solid electrolytes since these compounds are electronic insulators. The target surface would be charged with the same polarity as that of the ions in the plasma, and the sputtering plasma would rapidly break down. 

Another disadvantageous phenomenon in TFTs is the photoconductivity of a-Si H [626]. Electrons and holes are photogenerated and recombine at the back surface (gate insulator). The photocurrent reduces the on/off ratio of the TFT. Illumination, however, cannot always be avoided, e.g., in active matrix displays. A way of circumventing this is to make the a-Si H as thin as possible. 

Therefore, it must be supposed that the continuous electron emission from the surface of insulating film on the cathode will be existed by the ion bombardment. The work function for the electron emission from the surface of insulating film may be larger than that from the metal surface. Thus the cathode fall in front of the insulating film must be larger than that of metal surface. At a high discharge frequency where the inertia of an... 

A PEFC consists of two electrodes in contact with an electrolyte membrane (Fig. 14.7). The membrane is designed as an electronic insulator material separating the reactants (H2 and 02/air) and allowing only the transport of protons towards the electrodes. The electrodes are constituted of a porous gas diffusion layer (GDL) and a catalyst (usually platinum supported on high surface area carbon) containing active layer. This assembly is sandwiched between two electrically conducting bipolar plates within which gas distribution channels are integrated [96]. 

Figure 18. Discharge mechanism of a Li—SOCij ceii. The cell can operate until the surface of the carbon cathode is fully covered by electronically insulating LiCI and S discharge products. The Li—SO2 cell is also a soluble cathode system with a cell construction similar to that of the Li—SOCI2 cell. It follows a similar discharge reaction where the reaction product is L1S204.

Electrolyte solvents decompose reductively on the carbonaceous anode, and the decomposition product forms a protective film. When the surface of the anode is covered, the film prevents further decomposition of the electrolyte components. This film is an ionic conductor but an electronic insulator. 

Visual detection of surface layers on cathodes using microscopy techniques such as SFM seems to be supportive of the existence of LiF as a particulate-type deposition.The current sensing atomic force microscope (CSAFM) technique was used by McLarnon and co-workers to observe the thin-film spinel cathode surface, and a thin, electronically insulating surface layer was detected when the electrode was exposed to either DMC or the mixture FC/DMC. The experiments were carried out at an elevated temperature (70 °C) to simulate the poor storage performance of manganese spinel-based cathodes, and degradation of the cathode in the form of disproportionation and Mn + dissolution was ob-served. °° This confirms the previous report by Taras-con and co-workers that the Mn + dissolution is acid-induced and the electrolyte solute (LiPFe) is mainly responsible. 

As shown by Ruderman and Kittel (77) and Bloembergen and Rowland (78), Aij in a solid is dependent on the nature of the energy bands in the solid. For metals A is proportional to the product of the square of the electron density of Fermi surface electrons at the nucleus and the effective mass, and decreases as the inverse cube of the internuclear distance. Insulators have been treated by the energy band method (78) and by a molecular method (79) where each atom is considered to be bonded to its nearest neighbors. Unfortunately, both of these methods involve approximations in the evaluation of An which are quite crude at present. 

As illustrated in Figure 3.2, when the microelectrode is distant from the surface by several electrode diameters, a steady-state current, ij., is observed at the tip. The magnitude of the current is the same as that observed for a microdisk in a conventional experiment. When the tip is near a surface, the tip current, ij, differs from ij.oc, and depends on both the distance between the surface and tip, and the chemical nature of the surface. If the interfacial assembly efficiently blocks electron transfer, i.e. it is an electronic insulator, the mediator will not be regenerated, thus causing to be less than unity. If the Red species becomes re-oxidized at... 

In conclusion, the above surface film formation processes are expected to occur with all types of the carbons mentioned (including doped diamonds and fullerenes) in nonaqueous solvents containing metal ion salts. Hence, when carbon electrodes are utilized for electroanalysis or electrosynthesis in such solutions at low potentials, they should be considered as modified electrodes covered with surface films that are, at least partially, electronic insulators (but may be ion conductors). 

We can assume that as the surface films formed on active metals in solutions reach a certain thickness, they become electronic insulators. Hence, any possible electrical conductance can be due to ionic migration through the films under the... 

An insulator, on the other hand, has an ill-defined Fermi level which does not equilibrate with the spectrometer. Instead, the vacuum level of the insulator (E ) aligns with the local electrostatic potential surrounding its surface. An insulator more than a micron thick (which is the case for most catalyst samples analyzed by XPS) will not be within the local potential of the metal sample holder. The insulator will be separated from the spectrometer vacuum level (EJ) by some voltage (Vp) (30). This voltage will depend on the geometry of the sample holder and on the energy and flux of electrons from the x-ray source, the flood gun, the sample itself, and all other sources within the chamber. The potential Vp cannot be reliably measured. 

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