Alumina reversible cells

In early research, tetracyanoethylene (TCNE) had been used as a cathode material to circumvent the corrosion problems prevalent in many existing electrochemical couples using the Na/p"-alumina half cells [24]. TCNE exhibited reversible electrochemical characteristics with a potential over 3.0 V versus Na/Na" and an energy density of 620 Wh kg Recently, most of attention has been focused on carbonyl compounds owing to their favorable electrochemical performance for sodium ions. Despite of carbonyl compounds, other organic electrode materials such as conducting polymers and free radicals have also been characterized for sodium storage. 

Aluminum. All primary aluminum as of 1995 is produced by molten salt electrolysis, which requires a feed of high purity alumina to the reduction cell. The Bayer process is a chemical purification of the bauxite ore by selective leaching of aluminum according to equation 35. Other oxide constituents of the ore, namely siUca, iron oxide, and titanium oxide remain in the residue, known as red mud. No solution purification is required and pure aluminum hydroxide is obtained by precipitation after reversing reaction 35 through a change in temperature or hydroxide concentration the precipitate is calcined to yield pure alumina. 

Heinze modified the technique by purifying the solvent by transferring it under high vacuum from the electrochemical cell to superactive alumina in a separate vessel [27, 28]. Such sophisticated methods made it possible to generate reversible polyanions up to octaanions of aromatic and olefinic hydrocarbons [27, 28, 30-33]. 

Consider the battery in Fig. 7.18. The sodium beta alumina barrier allows sodium ions formed at the anode to Row across to the sulfur compartment, where, together with the reduction products of the sulfur, U forms a solution of sodium trisulfide in the sulfur. The latter is held at 300 CC to keep it molten. The sodium beta alumina also acts like an electronic insulator to prevent short circuits, and it is inert toward both sodium and sulfur. The reaction is reversible. At the present state of development, when compared with lead storage cells, batteries of this sort develop twice the power on a volume basis or four times the power on a weight basis. 

Unfortunately, -alumina cannot be used as a fuel cell electrolyte since its oxygen ions are immobile and so any oxygen electrodes in the corresponding galvanic cells will not be reversible. However, under open circuit conditions, /1-alumina has been successfully used as an oxygen potential sensor. In this mode of application, the oxygen electrode equilibrates with the Na+ ions of the electrolyte as follows... 

During the PEVD process, a chemical redox reaction takes place and the whole PEVD system can he viewed as a chemical reactor where the reactants are distributed over both the source and sink sides. According to the previous discussion, the driving force for this PEVD process can be solely provided by a dc electric potential, so that isolation of the source and sink vapor phases is not necessary. Consequently, the PEVD process is equivalent to physically moving a solid phase Na COj through another solid phase (Na+-[3"-alumina) by electric energy. Furthermore, it should be pointed out that the overall cell reaction in this PEVD system is reversible. 

The concept that the removal of an undesired reaction product by selective adsorption from the reaction zone of an equilibrium-controlled reaction increases the conversion and the rate of formation of the desired component (based on Le Chatelier s principle) was used to develop a novel PSA process concept called SERP for direct production of fuel cell-grade hydrogen by steam reforming of methane (CH4 + 2H20 44 C02 + 4H2).57 61 The concept uses a physical admixture of a reforming (noble metal on alumina) catalyst and a chemisorbent (K2C03 promoted hydrotalcite), which selectively and reversibly chemisorbs C02 from a gas at a temperature of -450 °C in the presence of steam. The cyclic SERP steps consisted of the following ... 

Super-dry electrolyte media, e.g., dry CHjCN under vacuum-line conditions, are used to study the second oxidation of (h -C5H5)jNi the wave is highly reversible at RT compared to the — 40°C required for reversibility if the solvent is less stringently prepared (Fig. D". Addition of activated alumina directly to the electrochemical cell and stirring the solution briefly before making the scan can be used to study the cation radical/dication redox behavior of aromatics . 

M. Z. Munshi and P. S. Nicholson, Demonstration of medium temperature, one atmosphere reversible H2/O2 fuel and steam electrolysis cell operation using polycrystalline H3O p/p"-alumina. Solid State Ionics 23 (1987) 203-9. 

Though the conductivity of the PEO-alkali metal complexes (10 ohm -cm at 140°C) is fairly low in comparison with inorganic solid electrolytes such as 8-alumina(Na), RbCui6l7Cli3 and RbAg4ls at the same temperature, this can be compensated for by the facile production of thin films typically 25-500 pm thick. A cell may thus consist of a lithium or lithium-based foil as anode, an alkali metal salt-PEO complex, such as (PEO)9 LiCFsSOs (25-50 pm), as the electrolyte, and a composite cathode (50-75 pm) containing a vanadium oxide (VeOis) as the active ingredient. The vanadium oxide is one of a number of insertion compounds that permits the physical insertion lithium ions reversibly into their structure and thus allows recharging of the cell. 

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