Molten membrane applications

For all hydrogen transport membranes, the partial pressure of hydrogen in the permeate must always be maintained at a partial pressure lower than that of the hydrogen in the source in order to maintain a net hydrogen flux from the source to the permeate [14]. For membrane applications with molten metal cooling fluids, in which upstream partial pressures in the retentate can be equivalent to only 2.7 x 10 Pa to 6.7 x lO" Pa (2.0 x lO" torr to 5.0 x 10 torr) [15], the partial pressure of hydrogen in the permeate must be kept exceptionally low. 

Liquid Membranes Several types of liquid membranes exist molten salt, emulsion, immobilized/supported, and hollow-fiber-contained liquid membranes. Araki and Tsukube (Liquid Membranes Chemical Applications, CRC Press, 1990) and Sec. IX and Chap. 42 in Ho and Sirkar (eds.) (op. cit., pp. 724, 764-808) contain detailed information and extensive bibliographies. 

There are six different types of fuel cells (Table 1.6) (1) alkaline fuel cell (AFC), (2) direct methanol fuel cell (DMFC), (3) molten carbonate fuel cell (MCFC), (4) phosphoric acid fuel cell (PAFC), (5) proton exchange membrane fuel cell (PEMFC), and (6) the solid oxide fuel cell (SOFC). They all differ in applications, operating temperatures, cost, and efficiency. 

Various types of fuel cells have been developed to generate power according to the applications and load requirements (Chaurasia, 2000). There are several types of electrolyte, which plays a key role in the different types of fuel cells. It must permit only the appropriate ions to pass between the anode and cathode. The main electrolyte types are alkali, molten carbonate, phosphoric acid, proton exchange membrane (PEM), and solid oxide. The first three are liquid electrolytes, the last two are solids. 

Fuel cells can be broadly classified into two types high temperature fuel cells such as molten carbonate fuel cells (MCFCs) and solid oxide polymer fuel cells (SOFCs), which operate at temperatures above 923 K and low temperature fuel cells such as proton exchange membrane fuel cells (PEMs), alkaline fuel cells (AFCs) and phosphoric acid fuel cells (PAFCs), which operate at temperatures lower than 523 K. Because of their higher operating temperatures, MCFCs and SOFCs have a high tolerance for commonly encountered impurities such as CO and CO2 (CO c)- However, the high temperatures also impose problems in their maintenance and operation and thus, increase the difficulty in their effective utilization in vehicular and small-scale applications. Hence, a major part of the research has been directed towards low temperature fuel cells. The low temperature fuel cells unfortunately, have a very low tolerance for impurities such as CO , PAFCs can tolerate up to 2% CO, PEMs only a few ppm, whereas the AFCs have a stringent (ppm level) CO2 tolerance. 

We summarize what is special with these prototype fast ion conductors with respect to transport and application. With their quasi-molten, partially filled cation sublattice, they can function similar to ion membranes in that they filter the mobile component ions in an applied electric field. In combination with an electron source (electrode), they can serve as component reservoirs. Considering the accuracy with which one can determine the electrical charge (10 s-10 6 A = 10 7 C 10-12mol (Zj = 1)), fast ionic conductors (solid electrolytes) can serve as very precise analytical tools. Solid state electrochemistry can be performed near room temperature, which is a great experimental advantage (e.g., for the study of the Hall-effect [J. Sohege, K. Funke (1984)] or the electrochemical Knudsen cell [N. Birks, H. Rickert (1963)]). The early volumes of the journal Solid State Ionics offer many pertinent applications. 

The excellent insulating and dielectric properties of BN combined with the high thermal conductivity make this material suitable for a huge variety of applications in the electronic industry [142]. BN is used as substrate for semiconductor parts, as windows in microwave apparatus, as insulator layers for MISFET semiconductors, for optical and magneto-optical recording media, and for optical disc memories. BN is often used as a boron dopant source for semiconductors. Electrochemical applications include the use as a carrier material for catalysts in fuel cells, electrodes in molten salt fuel cells, seals in batteries, and BN coated membranes in electrolysis cells for manufacture of rare earth metals [143-145]. 

Mishra, B., D.L. Olson and P.D. Ferro, 1994, Application of ceramic membranes in molten salt processing of radioactive wastes, in Proc. 123rd Ann. Meeting of Minerals, Metals, and Mater. Soc., San Francisco, USA, p. 233. 

The reversibility of the Pt(02)IZr02(Ca0) membrane oxygen electrode in molten KCl-NaCl has been studied in a number of works. Thus, the authors of Refs. [65, 236, 237] reported the application of the electrode with a Zr02(CaO) solid electrolyte membrane for the potentiometric investigation of some heterogeneous acid-base equilibria and the construction of the row of cation acidity in the KCl-NaCl melt. 

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