Conductivity directed metal oxidation

Direct tunnel conduction between metal oxide particles... 

Oxides of various metals are a broad class of electrode materials useful in many electrochemical processes (Trasatti, 1980-1981). The surfaces of practically all metals (both base and noble) become covered by layers of chemisorbed oxygen upon anodic polarization. The composition and properties of these layers depend on potential, on the electrolyte, and on the electrolysis conditions. They are often rather thick and have a distinct phase character, so that the metal electrode is converted to a typical oxide electrode. One can also make electrodes directly from oxides deposited in some way or other on various conducting substrates. 

It is known that some spinel-structured 3d-metal oxides are good catalysts for many processes involving electron transfer [12]. However, their low conductivity does not allow for the direct use in the electrode of the battery, and grafting them onto the carbon matrix is also very difficult technical problem. It was found recently that this problem could be solved indirectly, creating the spinel catalytic centers on the surface of carbon by means of adsorption of some 3d-metal complexes on the graphite surface followed by subsequent pyrolysis at certain temperatures [13,14],... 

While XAS techniques focus on direct characterizations of the host electrode structure, nuclear magnetic resonance (NMR) spectroscopy is used to probe local chemical environments via the interactions of insertion cations that are NMR-active nuclei, for example lithium-6 or -7, within the insertion electrode. As with XAS, NMR techniques are element specific (and nuclear specific) and do not require any long-range structural order in the host material for analysis. Solid-state NMR methods are now routinely employed to characterize the various chemical components of Li ion batteries metal oxide cathodes, Li ion-conducting electrolytes, and carbonaceous anodes.Coupled to controlled electrochemical in-sertion/deinsertion of the NMR-active cations, the... 

The remarkable insensitivity ofback-ET rates to pH-induced shifts in the conduction-band edge raises the question of why cb responds to pH in the first place. Experimentally, conduction-band edges for metal oxide semiconductor electrodes, including Sn02 electrodes, are observed to shift in the negative direction... 

Accordingly, correlations are often found between activity, selectivity and the concentration of cations in specific oxidation states, e.g. V4+ in V205. The improvement of selective catalytic qualities of metal oxides by addition of modifiers, or by combination in mixed oxides may hence be explained by stabilization of the essential cation in the proper oxidation state. In some cases, stabilization of a (partially) reduced cation appears to yield the most effective catalyst however, more often it is the higher oxidation state that should be maintained, and accordingly the role attributed to a second cation often concerns facilitating the reoxidation, for instance, by direct electron transfer between the cations or, in general, by increasing electron conductivity. 

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