Sodium anodes

Rechargeable cells with a sulfur cathode, a sodium anode, and a solid electrolyte such as yS-alumina have been much investigated [8[. They are operated at about 618 K, temperature at which sodium and sulfur are both liquid. 

The sodium/sulfur battery operates around 570-620 K and consists of a molten sodium anode and a liquid sulfur cathode separated by a solid P-alumina electrol5he (see Section 27.3). The cell reaction is ... 

Figure 5. Sodium anode assembly. Left hot-pressed /3-alumina cup. Right complete anode compartment comprising two mating cups and a fiU-tube...

The ZEBRA cell, which is under development by the General Electric Co., uses a molten-sodium anode and a solid p,p"-alumina solid electrolyte as in the sodium-sulfur cell, but the positive electrode is large-surface-area nickel rather than molten sulfur with a large-surface-area current collector. The electrolyte on the cathode side of the ZEBRA solid electrolyte is an aqueous NaAlCLt containing NaCl and Nal as well as a little FeS. The FeS and Nal are added to limit growth of the Ni particles and to aid the overall cathode reaction, which is... 

The cell is based upon a liquid sodium anode and liquid sulphur cathode, separated by a beta alumina ceramic-type electrolyte which is an electronic insulator, but through which sodium ions diffuse rapidly at 300-400 C. During discharge the reaction 2Na + 5S —> Na S. leads to an open circuit voltage (GCV) of 2.08V. Continued reaction beyond Na S results in the formation of lower polysulphides in the range Na - Na S (OCV 1.78V), after which solid separates out. The polysulphides... 

The sodium-sulfur (NaS) battery, (o) A single cell with a Liquid sodium anode and a Liquid sulfur cathode separated by a solid /3-AI2O3 electrolyte, (b) Multiple NaS cells in a vacuum-insulated box. 

The cell voltage of a beta cell, with a central liquid sodium anode, discharged at a 19-A rate is shown in Figure 11.4a. Discharge of the cell involves ionization of sodium atoms at the anode... 

Figure 29.1 Schematic modei of a sodium-suiphur battery which uses a sodium-p-aiumina soiid eiectroiyte as the separator between iiquid eiectrodes (sodium anode and suiphur cathode) operating temperature 300-400°C (Courtesy of Ford Motors)...

Sodium hydroxide is manufactured by electrolysis of concentrated aqueous sodium chloride the other product of the electrolysis, chlorine, is equally important and hence separation of anode and cathode products is necessary. This is achieved either by a diaphragm (for example in the Hooker electrolytic cell) or by using a mercury cathode which takes up the sodium formed at the cathode as an amalgam (the Kellner-Solvay ceW). The amalgam, after removal from the electrolyte cell, is treated with water to give sodium hydroxide and mercury. The mercury cell is more costly to operate but gives a purer product. 

The aqueous solution of sodium chlorate(I) is an important liquid bleach and disinfectant. It is produced commercially by the electrolysis of cold aqueous sodium chloride, the anode and cathode products being mixed. The sodium chloride remaining in the solution does not usually matter. There is evidence to suggest that iodic(I) acid has some basic character... 

Chloiine is pioduced at the anode in each of the three types of electrolytic cells. The cathodic reaction in diaphragm and membrane cells is the electrolysis of water to generate as indicated, whereas the cathodic reaction in mercury cells is the discharge of sodium ion, Na, to form dilute sodium amalgam. 

Fig. 7. Mercury cathode electroly2er and decomposer (11) 1, brine level 2, metal anodes 3, mercury cathode, flowing along baseplate 4, mercury pump 5, vertical decomposer 6, water feed to decomposer 7, graphite packing, promoting decomposition of sodium amalgam 8, caustic Hquor exit 9, denuded mercury 10, brine feed 11, brine exit 12, hydrogen exit from decomposer 13, chlorine gas space 14, chlorine exit 15, wash water.

Electrolytic Preparation of Chlorine and Caustic Soda. The preparation of chlorine [7782-50-5] and caustic soda [1310-73-2] is an important use for mercury metal. Since 1989, chlor—alkali production has been responsible for the largest use for mercury in the United States. In this process, mercury is used as a flowing cathode in an electrolytic cell into which a sodium chloride [7647-14-5] solution (brine) is introduced. This brine is then subjected to an electric current, and the aqueous solution of sodium chloride flows between the anode and the mercury, releasing chlorine gas at the anode. The sodium ions form an amalgam with the mercury cathode. Water is added to the amalgam to remove the sodium [7440-23-5] forming hydrogen [1333-74-0] and sodium hydroxide and relatively pure mercury metal, which is recycled into the cell (see Alkali and chlorine products). 

Success in the chlorine industry led to the incorporation of DSA in sodium chlorate [7775-09-9] NaClO, manufacture. The unique stmctural characteristics of the anode allowed for innovative designs in ceU hardware, which in turn contributed to the extensive worldwide expansion of the sodium chlorate industry in the 1980s. 

An expandable anode involves compression of the anode stmcture using cHps during cell assembly so as not to damage the diaphragm already deposited on the cathode (Eig. 3a). When the cathode is in position on the anode base, 3-mm diameter spacers are placed over the cathode and the cHps removed from the anode. The spring-actuated anode surfaces then move outward to bear on the spacers, creating a controlled 3-mm gap between anode and cathode (Eig. 3b). This design has also been appHed to cells for the production of sodium chlorate (22). 

Other Metals. AH the sodium metal produced comes from electrolysis of sodium chloride melts in Downs ceUs. The ceU consists of a cylindrical steel cathode separated from the graphite anode by a perforated steel diaphragm. Lithium is also produced by electrolysis of the chloride in a process similar to that used for sodium. The other alkaH and alkaHne-earth metals can be electrowon from molten chlorides, but thermochemical reduction is preferred commercially. The rare earths can also be electrowon but only the mixture known as mischmetal is prepared in tonnage quantity by electrochemical means. In addition, beryIHum and boron are produced by electrolysis on a commercial scale in the order of a few hundred t/yr. Processes have been developed for electrowinning titanium, tantalum, and niobium from molten salts. These metals, however, are obtained as a powdery deposit which is not easily separated from the electrolyte so that further purification is required. 

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