High-temperature electrolyzer

Dutta S, Morehouse JH, Khan JA (1977) Numerical analysis of laminar flow and heat transfer in a high temperature electrolyzer. Int J Hydrogen Energy 22 883-895... 

Cost was an important consideration for us in the development of our experimental solar hydrogen system, so a PEM electrolyzer was discounted in favor of a low pressure alkaline tank electrolyzer. High pressure and high temperature electrolyzers were also evaluated. These were discounted due to higher amounts of energy consumed and the need for more intensive monitoring of the system. 

In parallel, in 1900, Walther Hermann Nemst (1864-1941), a German physicist and chemist, developed the high-temperature electrolyte YSZ , based on zirconium dioxide (Zr02) stabilized by yttrium oxide (Y2O3) at a 15% mass ratio. The foundations were lain for high-temperature electrolyzers and batteries. 

With the exception of Japan, there was httle research carried out on high-temperature electrolyzers during the 1990s. However, development has resumed since the beginning of this century, and is gathering momentum, more or less... 

Many electrochemical devices and plants (chemical power sources, electrolyzers, and others) contain electrolytes which are melts of various metal halides (particularly chlorides), also nitrates, carbonates, and certain other salts with melting points between 150 and 1500°C. The salt melts can be single- (neat) or multicomponent (i.e., consist of mixtures of several salts, for their lower melting points in the eutectic region). Melts are highly valuable as electrolytes, since processes can be realized in them at high temperatures that would be too slow at ordinary temperatures or which yield products that are unstable in aqueous solutions (e.g., electrolytic production of the alkali metals). 

Murray J.N., Yaffe M.R., Testing aqueous caustic electrolyzers at high temperatures, Int. ]. Hydrogen Energ., 4,193-204,1979. 

Figure 7.17 shows a summary of the available conditions of water electrolysis [72]. For each configuration there exists a range of performance. Conventional electrolyzers, which nevertheless are still the most common in the current production of H 2 on the intermediate and small scale, show high overpotential and a relatively small production rate. Membrane (SPE) and advanced alkaline electrolyzers show very similar performance, with somewhat lower overpotential but a much higher production rate. Definite improvements in energy consumption would come from high temperature (steam) electrolysis, which is, however, still far from optimization because of a low production rate and problems of material stability. 

Manganese also is produced by electrolysis of fused salt. In one such process, the reduced MnO is blended to molten calcium fluoride and lime. The latter is used to neutralize silica in the ore. The fused composition of these salts is electrolyzed at 1,300°C in an electrolytic cell made up of high temperature ceramic material, using a carbon anode and a cathode consisting of iron bars internally cooled by water. 

Metallic uranium can be prepared from its oxides or hahdes by reduction at high temperature. Uranium dioxide, UO2, or other oxides such as UO3 or UsOs may be reduced to uranium metal by heating with carbon, calcium or aluminum at high temperatures. Similarly, uranium tetrafluoride or other halides can be reduced to metal by heating with sodium, potassium, calcium, or magnesium at high temperatures. Alternatively, uranium tetrafluoride mixed with fused alkali chlorides is electrolyzed to generate uranium metal. 

Vanadium metal is prepared from pentoxide, V2O5, by reduction with calcium at elevated temperatures. Presence of iodine lowers calcium reduction temperature to 425°C because of heat of formation of calcium iodide. Pentoxide also may be converted to the trichloride, VCI3, and the trichloride reduced with magnesium metal or magnesium-sodium mixture at high temperatures to form high purity ductile metal. Alternatively, a fused mixture of vanadium chloride, sodium chloride, and hthium chloride may be electrolyzed to produce the metal in high purity. 

PCEC Protonic Ceramic Electrolyzer Cell (High Temperature, H+ conduction)... 

Should this prove insufficient, or in cases where Joule s heat cannot be used for some reasons, special heating devices must be employed. Heating devices installed outside the electrolyzer are suitable for open containers and not too high temperatures. Such heaters are installed before the first cell in an arrangement of cascades with a circulation of the electrolyte. The electrolytic cell can also be provided with a steam heating jacket. 

Summary Iron-II-oxide can be readily prepared by first, electrolyzing a solution of pickling salt using iron electrodes. During the electrolysis process, a messy precipitate of mixed hydrated iron oxides is formed. Thereafter, this precipitate is collected by filtration, and then dried. The dried messy mass is then dried in a desiccator under mild heat for 12 to 24 hours to facilitate formation of the iron-II-oxide. For the preparation of iron-III-oxide, the same electrolysis process is used to form the initial messy mass, and this mass is then collected by filtration and dried in the usual manner. However, instead of drying this mass in a desiccator, it is roasted at high temperature for several hours to facilitate formation of iron-III-oxide, which is formed by the oxidization of the iron-II-oxides. 

---