Cathodic reduction of vanadium

The current efficiencies for these salt productions are typically greater than 98%. Cathodic reduction can also be used for the production of metal salts, e.g. vanadium(II) formate by the cathodic reduction of vanadium(V). 

S.3.3 Cathodic reduction of vanadium in NaCl-KCl-VCl3 melts... 

The cathodic reduction of vanadium in VCl3-NaCl-KCl melts is a two-step process a one-electron reduction and a two-electron V process. Under a constant applied current both stages are diffusion-controlled. During voltammetry measurements the mechanism of the electrode reactions remains unchanged at polarization rates below about 200mV/s. The diffusion coefficients of V(II) and V(III) ions were determined from the results of cyclic voltammetry measurements. It was also found that tungsten reacts with V(in) ions and thus cannot be used as electrode material for studying electrochemical... 

The authors react Fe with a low-grade, acid, sub-bituminous coal (SBC) slurry to produce carbon dioxide and Fe. The latter ion can then be harnessed as the anode of the cell, with the oxidation reaction Fe to Fe coupled to the reduction of vanadium dioxide at the cathode of the fuel cell. The first primitive device is said to be 7% efficient, but that is based on the fuel calorific value, asserted to be irrational in this book and in (Barclay, 2002). An alternative catalyst is mooted to start the path to improvement. The fate of the used slurry is not discussed. It could be burnt or buried, depending on economics. 

Alternate method Electrolysis of V3O5-containing phosphate melts, with cathodic reduction to vanadium phosphides [M. Chene, Comptes Rendus Hebd. Seances Acad. Sci. 2W, 1144 (1939) Ann. chimie [11], 15, 272 (1941)]. 

Wang and coworkers synthesized the PPy/V O hollow micro-spherical hybrid by in-situ polymerization method as cathode material for LIB. Compared to bare electrode (32.7% retention), this PPy/oxide cathode exhibits slightly reduced capacity and substantially improved cyclabil-ity (89.1% retention after 50 cycles), and it was demonstrated that the reduction in the capacity is due to the partial reduction of vanadium during the polymerization of polymer [41],... 

It is interesting to note that even at maximum current load (tens of amperes per square centimeter) alkali metal does not form a separate phase on the cathode. There is only a sharp potential shift to -2.3 to -2.5 V on the chronopotentiograms, followed by gradual change of potential toward positive values caused by secondary reduction of vanadium in the melt (Figure 4.5.19d). 

In 1898, Cowper-Coles 2 claimed to have successfully effected the electrolytic reduction of an acid solution of vanadium pentoxide to metallic vanadium, but the product was subsequently shown by Fischer 3 to have been a deposit of platinum hydride. Fischer, in a series of over three hundred experiments, varied the temperature, current density, cathode material, concentration, electrolyte, addition agent, and construction of cell, but in not one instance was the formation of any metallic vanadium observed. In most cases reduction ceased at the tetravalent state (blue). At temperatures above 90° C. reduction appeared to proceed to the divalent state (lavender). The use of carbon electrodes led to the trivalent state (green), but only lead electrodes produced the trivalent state at temperatures below 90° C. Platinum electrodes reduced the electrolyte to the blue vanadyl salt below 90° C. using a divided cell and temperatures above 90° C. the lavender salt was obtained. 

Certain systems which behave reversibly in the equilibrium state exhibit considerable polarization in the course of electrolytic reduction examples are the conversion of 5-valent vanadium to the 4-valent state, and of the latter to the 3-valent condition, the reduction of 6- to 5- and of 5- to 3-valent molybdenum, and the reduction of 6-valent to 5-valent tungsten. There is reason to believe, however, that in all these cases the abnormal behavior is to be attributed to the presence of oxide films on the cathode by producing a partial blocking of the surface, these oxide films increase the effective c.d., so that the potential rises. Considerable polarization, accompanied by oxide-film formation, occurs in the reduction of chromate to chromic ions, but it is not certain how far this system is reversible. 

By reduction of the trioxide with carbon.2 (5) By the thermite reduction process.3 A product which was 99 per cent pure has been obtained by this method or by reduction with vanadium carbide. (6) By electrolysis of a solution of the trioxide in fused calcium vanadate.4 The anode is made of carbon and the cathode is prepared by pulverizing ferro-vanadium and pressing the powder into a cone-shaped form. The current density used is 4.5 amperes per square inch of anode surface. 

This solution is subjected to electrolytic reduction in a cell containing a clay cylinder diaphragm the current is 2-3 amp. at 10 V. (the procedures are those described on pp. 1277 and 1284). The electrolysis is continued until the electrolyte at the cathode shows the pure green color of vanadium (III). The best electrodes are those made of platinum sheet. 

Alfonsi (9,10,11,12,13) has carried out an extensive investigation of the controlled-potential separation and determination of antimony in alloys containing combinations of lead, tin, bismuth, and copper. Tanaka (14, 15), working mainly with synthetic samples, reports conditions for the separation of antimony from gold, silver, mercury, copper, bismuth, cadmium, zinc, and vanadium in a variety of common electrolytes. Very recently, Dunlap and Shults (18) have developed two coulometric procedures which permit the determination of antimony in each of its oxidation states as well as the total antimony present. After pre-reduction of antimony (V) with hydrazine hydrate, the antimony (III) is reduced to the amalgam at a mercury cathode with a potential of —0.28 V vs. SCE in a supporting electrolyte 0.4 m in tartaric add and 1 m in hydrochloric acid. In the... 

Sulphates, which form part of the ash from the combustion of many fuels, are not harmful to high-alloy steels, but can become so if reduction to sulphide occurs. This leads to the formation of low melting point oxide-sulphide mixtures and to sulphide penetration of the metal. Such reduction is particularly easy if the sulphate can form a mixture of low melting point with some other substance. Reduction can be brought about by bad combustion, as demonstrated by Sykes and Shirley , and it is obviously important to avoid contact with inefficiently burnt fuels when sulphate deposits may be present. Reduction can also be brought about in atmospheres other than reducing ones and the presence of chlorides or vanadium pentoxide has been shown to be sufficient to initiate the reaction. It has also been shown that it can be initiated by prior cathodic polarisation in fused sodium sulphate. The effect of even small amounts of chloride on oxidation in the presence of sulphate is illustrated in Fig. 7.33 . 

Cathodic stripping voltammetry has been used [807] to determine lead, cadmium, copper, zinc, uranium, vanadium, molybdenum, nickel, and cobalt in water, with great sensitivity and specificity, allowing study of metal specia-tion directly in the unaltered sample. The technique used preconcentration of the metal at a higher oxidation state by adsorption of certain surface-active complexes, after which its concentration was determined by reduction. The reaction mechanisms, effect of variation of the adsorption potential, maximal adsorption capacity of the hanging mercury drop electrode, and possible interferences are discussed. 

Leifer et al. [105]. used Li MAS NMR to study the strucmre of lithiated silver vanadium oxide, Liy4g2V40n, where x = 0.72,2.13, and 5.59. This compound is used in biomedical applications as a primary battery, particularly as the power source for implantable cardiac defibrillators (ICDs). Silver vanadium oxide is a vanadium bronze with semiconducting properties. It has been used successfully as a cathode material in the battery of ICDs due to its high rate capability and its high theoretical capacity (315 mAh/g) to 2 V. Electrochemical and structural studies of the average structure were performed by various authors who concluded that the systems undergoes a multistep reduction mechanism and forms silver metal in the early stage of the overall reaction [106-108]. 

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