Molten hydroxide electrolyte

Calcium is produced by electrolysis in molten hydroxide electrolytes or by a thermal process. In the case of electrolysis in molten hydroxide electrolyte, hydrogen evolution is likely to be an important cathodic side reaction. 

Molten KOH or NaOH is used as the electrolyte, which is contained within a metallic container. The metallic container acts as the cathode and a carbon rod dipped into electrolyte acts as both fuel and anode of the cell. Molten hydroxide electrolytes possess advantageous features such as high ionic conductivity, low overpotential and high carbon oxidation rate with a low operation temperature of 600°C for DCFC, which make its components fabrication economical. The dominant reaction product would be CO2 instead of CO. The ceU reactions are given as follows ... 

Zecevic, Strahinja, Edward M. Patton and Parviz Parhami (2004) Direct carbon fuel cell with molten hydroxide electrolyte, Proc. 2nd International Conference on Fuel Cell Science, Engineering and Technology, June 14-16, 2004, ASME, Rochester New York, USA. 

The poor efficiencies of coal-fired power plants in 1896 (2.6 percent on average compared with over forty percent one hundred years later) prompted W. W. Jacques to invent the high temperature (500°C to 600°C [900°F to 1100°F]) fuel cell, and then build a lOO-cell battery to produce electricity from coal combustion. The battery operated intermittently for six months, but with diminishing performance, the carbon dioxide generated and present in the air reacted with and consumed its molten potassium hydroxide electrolyte. In 1910, E. Bauer substituted molten salts (e.g., carbonates, silicates, and borates) and used molten silver as the oxygen electrode. Numerous molten salt batteiy systems have since evolved to handle peak loads in electric power plants, and for electric vehicle propulsion. Of particular note is the sodium and nickel chloride couple in a molten chloroalumi-nate salt electrolyte for electric vehicle propulsion. One special feature is the use of a semi-permeable aluminum oxide ceramic separator to prevent lithium ions from diffusing to the sodium electrode, but still allow the opposing flow of sodium ions. 

Good descriptions of the production of aluminum can be found in the literature (Grjotheim etal. [7], Grjotheim and Welch [8], Grjotheim and Kvande [9], Burkin [10], and Peterson and Miller [11]). Referring to Fig. 2 [12], the first step in the production of aluminum from its ore ( bauxite ) is the selective leaching of the aluminum content (present as oxides/hy dr oxides of aluminum) into hot concentrated NaOH solution to form sodium aluminate in solution. After solution purification, very pure aluminum hydroxide is precipitated from the cooled, diluted solution by addition of seed particles to nucleate the precipitation. After solid-liquid separation the alumina is dried and calcined. These operations are the heart of the Bayer process and the alumina produced is shipped to a smelter where the alumina, dissolved in a molten salt electrolyte, is electrolyt-ically reduced to liquid aluminum in Hall- Heroult cells. This liquid aluminum,... 

The history of Molten Carbonate Fuel Cell (MCFC) can be traced back to the late nineteenth century when W.W. Jacques had produced his carbon-air fuel cell, a device for producing electricity from coal. This device used an electrolyte of molten potassium hydroxide at 400-5(X) °C in an iron pot [94]. Jacques suggested to replace molten alkali electrolytes with molten salts such as carbonates, silicates, and borates. 

Lithium Chloride. Lithium chloride [7447- 1-8], LiCl, is produced from the reaction of Hthium carbonate or hydroxide with hydrochloric acid. The salt melts at 608°C and bods at 1382°C. The 41-mol % LiCl—59-mol % KCl eutectic (melting point, 352°C) is employed as the electrolyte in the molten salt electrolysis production of Hthium metal. It is also used, often with other alkaH haHdes, in brazing flux eutectics and other molten salt appHcations such as electrolytes for high temperature Hthium batteries. 

Lithium Iodide. Lithium iodide [10377-51 -2/, Lil, is the most difficult lithium halide to prepare and has few appHcations. Aqueous solutions of the salt can be prepared by carehil neutralization of hydroiodic acid with lithium carbonate or lithium hydroxide. Concentration of the aqueous solution leads successively to the trihydrate [7790-22-9] dihydrate [17023-25-5] and monohydrate [17023-24 ] which melt congmendy at 75, 79, and 130°C, respectively. The anhydrous salt can be obtained by carehil removal of water under vacuum, but because of the strong tendency to oxidize and eliminate iodine which occurs on heating the salt ia air, it is often prepared from reactions of lithium metal or lithium hydride with iodine ia organic solvents. The salt is extremely soluble ia water (62.6 wt % at 25°C) (59) and the solutions have extremely low vapor pressures (60). Lithium iodide is used as an electrolyte ia selected lithium battery appHcations, where it is formed in situ from reaction of lithium metal with iodine. It can also be a component of low melting molten salts and as a catalyst ia aldol condensations. 

The electrolyte salt must be processed to recover the ionic plutonium orginally added to the cell. This can be done by aqueous chemistry, typically by dissolution in a dilute sodium hydroxide solution with recovery of the contained plutonium as Pu(OH)3, or by pyrochemical techniques. The usual pyrochemical method is to contact the molten electrolyte salt with molten calcium, thereby reducing any PUCI3 to plutonium metal which is immiscible in the salt phase. The extraction crucible is maintained above the melting point of the contained salts to permit any fine droplets of plutonium in the salt to coalesce with the pool of metal formed beneath the salt phase. If the original ER electrolyte salt was eutectic NaCl-KCl a third "black salt" phase will be formed between the stripped electrolyte salt and the solidified metal button. This dark-blue phase can contain 10 wt. % of the plutonium originally present in the electrolyte salt plutonium in this phase can be recovered by an additional calcium extraction stepO ). 

Metallic magnesium is produced by either chemical or electrolytic reduction of its compounds. In chemical reduction, first magnesium oxide is obtained from the decomposition of dolomite. Then ferrosilicon, an alloy of iron and silicon, is used to reduce the MgO at about 1200°C. At this temperature, the magnesium produced is immediately vaporized and carried away. The electrolytic method uses seawater as its principal raw material magnesium hydroxide is precipitated by adding slaked lime (Ca(OH)2, see Section 14.10), the precipitate is filtered off and treated with hydrochloric acid to produce magnesium chloride, and the dried molten salt is electrolyzed. 

Electrolyte Potassium hydroxide Polymer membrane Immobilized liquid molten carbonate Immobilized hquid phosphoric acid Ion exchange membrane Ceramic... 

Electrolyte Ion Exchange Membranes Mobilized or Immobilized Potassium Hydroxide Immobilized Liquid Phosphoric Acid Immobilized Liquid Molten Carbonate Ceramic Ceramic... 

Dr. William W. Jacques further explored the carbon approach in 1896. His fuel cells had a carbon rod central anode in the electrolyte of molten potassium hydroxide. He made a fuel cell system of 100 cylindrical cells, which produced as much as 1500 W. Francis T. Bacon worked on fuel cells to produce alkaline systems that did not use noble metal catalysts in the 1930s. He developed and built a 6 kW alkaline hydrogen-oxygen system in 1959. In the same year, Dr. Harry Ihrig introduced... 

In 1807 Sir Humphry Davy (1778-1829) devised an electrolysis apparatus that used electrodes immersed in a bath of melted sodium hydroxide. When he passed an electric current through the system, metallic sodium formed at the negative (cathode) electrode. He first performed this experiment with molten potassium carbonate to liberate the metal potassium, and he soon followed up with the sodium experiment. Today, sodium and some of the other alkali metals are still produced by electrolysis. The types of electrolytes may vary using a mixture of sodium chloride and calcium chloride and then further purifying the sodium metal. 

Although many commercial processes have heen developed since the first electrolytic isolation of Mg metal hy Davy and Faraday, and Bussy, hy chemical reduction, the principles of the manufacturing processes have not changed. At present, the metal is most commonly manufactured by electrolytic reduction of molten magnesium chloride, in which chlorine is produced as a by-product. In chemical reduction processes, the metal is obtained by reduction of magnesium oxide, hydroxide, or chloride at elevated temperatures. 

Molten carbonates are of interest because of their applications as electrolytes in molten salt fuel cells. The preparation, handling, physical and electrochemical properties, and important applications of molten alkali carbonates have been described at length [4,5], Li2C03, Na2C03, and K2C03 and various mixtures of these salts are the carbonates most frequently used as electrochemical solvents. The major impurities in alkali carbonates are alkali hydroxides and oxides produced through hydrolysis and dissociation ... 

Sodium chloride is plentiful as rock salt, but the solid does not conduct electricity, because the ions are locked into place. Sodium chloride must be molten for electrolysis to occur. The electrodes in the cell are made of inert materials like carbon, and the cell is designed to keep the sodium and chlorine produced by the electrolysis out of contact with each other and away from air. In a modification of the Downs process, the electrolyte is an aqueous solution of sodium chloride. The products of this chloralkali process are chlorine and aqueous sodium hydroxide. 

The OH" ions, which thus accumulate in the cathode compartment, are balanced by the Na+ ions, which are brought up by the electrolytic conduction. The commercial sodium hydroxide is obtained by evaporating the cathode solution to a high concentration of NaOH, in which NaCl is insoluble. The crystals of NaCl are removed in centrifugal filters, and the purified solution is evaporated until molten NaOH is left. This is poured into molds and allowed to solidify. If the total sodium hydroxide could be recovered, 40 grams would be realized for each faraday. It is the object of this experiment to determine what percentage of this ideal yield can be obtained in a simple cell. 

However, in many situations, water is hardly the ideal solvent. Take the electrolytic production of sodium metal, for exanple. If an aqueous solution of a sodium salt is taken in an electrolytic cell and a current is passed between two electrodes, then all that will happen at the cathode is the liberation of hydrogen gas there will be no electrodeposition of sodium (see Chapter 7). Hence, sodium cannot be electrowon from aqueous solutions. This is why the electrolytic extraction of sodium has taken place from molten sodium hydroxide, i.e., from a medium free of hydrogen. This ... 

Mixtures of molten alkali metal halides and hydroxides have a potential use in energy storage, where the relatively high value of the enthalpy of fusion is used. Molten carbonates of alkali metal halides are used as electrolytes in molten carbonate fuel cells. A big industrial field, where oxide melts are predominantly used, is the glass industry. Here, the high affinity of these melts to under-cooling and glass formation is exploited. 

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