Exotic Electrolytes

It is useful to compare the properties of the solvent-electrolyte systems we have considered so far with those of some more exotic electrolytes, meaning electrolytes with unusual properties not found in conventional electrolytes. Table 3.4 presents a compilation of ionic conductivities of some electrolytes that are extreme in some ways. The first four entries illustrate that an otherwise conventional electrolyte, TBAPFg in butyronitrile, can 

Ionic conductivity values for some exotic electrolytes 

Medium Electrolyte Electrolyte concentration (M) Temperature (°C) Conductivity (ohm cm ) Reference 

PC/EC/polyacryl onitrile gel Polyethylene-glycol-dimethylether (350 MW) with fumed silica LiTFSI, LiTiiflate, LiPF 5-15 wt.% 30 0.001-0.005 (18) 

The next entry is for Nafion, a proton-conducting fluorosulfonic acid ionomer material which in membrane form is widely used in PEM fuel-cell technology. The conductivity value quoted is for a fully hydrated membrane at an ambient temperature. Note that the conductivity is less than that of a comparable aqueous acid solution, for example 0.5 M sulfuric acid, but by a factor of only 3-4. Heavily hydrated Nafion membranes contain a lot of water, and consequently they behave a lot like aqueous acid solutions. The next three entries are for various gel and solid-polymer electrolytes containing lithium salts. All these material are membranes some contain some potentially volatile solvents, while others do not. Conductivities for these materials are low relative to true liquid solvents but they are still well within the range of usable values for electrochemical experiments. The semi-solid character of these materials, combined with their near-zero volatility (for solid-polymer electrolytes which do not contain volatile solvents), makes them suitable for use under high-vacuum conditions which makes them potentially useful for fabrication of electrochemical devices which are targeted for use in vacuum or under conditions which could otherwise result in solvent loss by evaporation. 

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Except for some exotic surface-mount technology (SMT/SMD) aluminum electrolytic capacitor types with solid electrolyte systems, in general, an aluminum electrolytic capacitor contains a wound capacitor element (the coil), impregnated with liquid electrolyte, connected to terminals, and sealed in a can (with a rubber plug at the end). The aluminum in the name,... 

Currently there is much interest in modifying the surfaces of commodity polymers with a view to either functionalising the surfaces to provide for more exotic materials or subjecting the treated surface to electroless or electrolytic plating to produce, for example, printed circuit boards. 

The research race aiming at solidification of DSC by replacing the liquid electrolytes with solid state materials such as conductive polymers and novel hole transport materials is still on. Tennakone disclosed the use of CuBr as an exotic hole transport material for solidification of DSC.,02) Such success would open up the possibility of a low-cost printing process to fabricate solid-state DSC. 

A galvanic cell consists of two electrodes, or metallic conductors, that make electrical contact with the contents of the cell, and an electrolyte, an ionically conducting medium, inside the cell. The electrolyte is typically an aqueous solution of an ionic compound, although advanced cells make use of all kinds of exotic materials (see Box 12.1). 

This chapter deals primarily with reactions in aqueous media, but the principles can be extended to fuel cells and other batteries like the high-temperature batteries with exotic nonaqueous electrolytes. 

Aluminum is present in most rocks and is the most abundant element in the earth s crust (eight percent by weight.) However, its isolation is very difficult and expensive to accomplish by purely chemical means, as evidenced by the high E° (-1.66 v) of the A13+/A1 couple. For the same reason, aluminum cannot be isolated by electrolysis of aqueous solutions of its compounds, since the water would be electrolyzed preferentially. And if you have ever tried to melt a rock, you will appreciate the difficulty of electrolyzing a molten aluminum ore Aluminum was in fact considered an exotic and costly metal until 1886, when Charles Hall (U.S.A) and Paul Herault (France) independently developed a practical electrolytic reduction process. 

Planar SOFCs are composed of flat, ultra-thin ceramic plates, which allow them to operate at 800°C or even less, and enable less exotic construction materials. P-SOFCs can be either electrode- or electrolyte- supported. Electrolyte-supported cells use YSZ membranes of about 100 pm thickness, the ohmic contribution of which is still high for operation below 900°C. In electrode-supported cells, the supporting component can either be the anode or the cathode. In these designs, the electrolyte is typically between 5-30 pm, while the electrode thickness can be between 250 pm - 2 mm. In the cathode-supported design, the YSZ electrolyte and the LSM coefficients of thermal expansion are well matched, placing no restrictions on electrolyte thickness. In anode-supported cells, the thermal expansion coefficient of Ni-YSZ cermets is greater than that of the YSZ... 

Ag or Pt wire quasi-reference electrodes are used for experiments conducted in the absence of deliberately added electrolyte or in exotic solvents where no established reference electrode couple exists. 

Among the exotic cells for the laboratory, flow cells (filter press cells) require an external loop with a pump to recirculate the electrolyte. Also, solid polymer electrode (SPE) cells have been described.16-19 The electrode material is deposited on both sides of a membrane (Nafion) or pressed against the membrane. An advantage is that the ionic conduction takes place in the membrane so that no supporting electrolyte is required. Moreover, this cell can work with gaseous reactants. 

While PThs show electroactivity in aqueous solutions [182-184], most studies have been carried out in non-aqueous solutions. Exotic solvents such as liquid SO2 [171,185] and ammonia [ 174] have also been used for a variety of reasons. As already mentioned, many different electrolytes have been used, which determine voltaminetric shapes as well as kinetics involved. 

Although contemporary systems and processes may be complex, the techniques and the content of this book stUl apply. But to maximize the value of our approach, you may need to create new definitions, characterize other properties, consider additional interactions that influence complex systems, implement coimections to molecular theory and statistical mechanics, and derive appropriate relations that are amenable to reliable modeling. In the past such characterizations were commonly done in terms of macroscopic measurables, but now molecular structure is being used to describe complex systems, including alternative-energy systems, biochemicals, colloids and interfaces, electrolytes, polymers, and exotic materials. 

In solid polymeric electrolytes, the polymers involved can be considered to be immobile nonaqueous solvents they have the chemical characteristics of typical nonaqueous solvents but are macroscopically immobile, unlike common nonaqueous solvents which are small molecules, free to move long distances. When considered from this viewpoint, one immediately sees that these systems are an exotic extension of nonaqueous solution chemistry, and that the classical chemical studies on the nature of solvation carried out on liquid electrolytes are extremely pertinent. 

There exist a variety of fuel cells. For practical reasons, fuel cells are classified by the type of electrolyte employed. The following names and abbreviations are frequently used in publications alkaline fuel cells (AFC), molten carbonate fuel cells (MCFC), phosphoric acid fuel cells (PAFC), solid oxide fuel cells (SOFC), and proton exchange membrane fuel cells (PEMFC). Among different types of fuel cells under development today, the PEMFC, also called polymer electrolyte membrane fuel cells (PEFC), is considered as a potential future power source due to its unique characteristics [1-3]. The PEMFC consists of an anode where hydrogen oxidation takes place, a cathode where oxygen reduction occurs, and an electrolyte membrane that permits the transfer of protons from anode to cathode. PEMFC operates at low temperature that allows rapid start-up. Furthermore, with the absence of corrosive cell constituents, the use of the exotic materials required in other fuel cell types is not required [4]. 

TSeT)2-halides combine high room temperature conductivity (o = 2500 Qcm ) with high thermal stability up to over 250 °C (see Fig. l).They are, however, practically insoluble even in exotic organic solvents. Only the donor TSeT itself shows limited solubility in high boiling polar solvents. Until recently pure 2 1 halides could only be made by electrocrystallization in nitrobenzene in the presence of an organic ammonium halide as electrolyte. Chemical pathways like oxidation with Cl2-gas or ferrous chloride lead to impure products mainly due to overoxidation of the donor. 

This class of electrolytes gives the technology of lithium primary batteries a special exotic attraction. The fluid electrolyte mixture acts as the media of transfer of electric charges between anode and cathode as described above. In addition it also contains the cathodic active substance, which is in direct contact to the anodic counterpart, the lithium metal, but nonetheless reacts separately in a distance from the anode at a cathodic support electrode by consumption of electrons from the outer circuit. This paradoxical behavior is possible because of the cathode s ability to create a... 

Another advantage of great importance relates to the system cost. There are very few standard chemicals that are cheaper than potassium hydroxide. It really is a very low-cost material the electrolyte cost of the AFC is thus far less than any other type - and it always will be. Also, the electrodes, particularly the cathode, can be made from nonprecious metals, and no particularly exotic materials are needed. The electrodes are thus considerably cheaper than other types of fuel cells, and there is no reason to suppose this will change in the near future either. 

One of the problems with anode-supported cells is that any difference in thermal expansion between anode and electrolyte becomes more significant than in conventional high-temperature SOFCs. For this reason many developers use porous nickel cermet anodes with interfacial regions made of NiA SZ doped with ceria. Operating at temperatures below about 700°C means that metallic bipolar plates can be used, and the lower the temperature, the less exotic the steel needs to be. Ferritic stainless steels can be used below about 600°C, and these have the advantage that they have a low thermal expansion coefficient. Conventional doped LSM-YSZ cathodes can be used but there is much development in progress to improve cathode materials as the cathode overpotentials become more significant as the cell temperatures are lowered. A recent review of cathode materials has been published by Ralph (2001). 

Hara [237] prepared perchlorate, sulfate, and acetate solutions containing Am02 free of Am by first extracting AmO from buffered 1m acetate (pH 3) solutions of Am(iii) and Am(v) into 0.1 M thenoyltrifiuoroacetone in isobutanol and back-extracting the Am(v) into an aqueous phase. More exotic methods for obtaining the AmOj ion in aqueous solution include dissolution of solid U3 Am04 in dilute perchloric acid or the electrolytic oxidation of Am(iii) in 2 M LilOj/O. M HIOj (pH 1.47) solution [3]. 

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