Vacuum evaporation, solid electrolytes

Fabrication techniques, especially the preparation of thin films of functional materials, have made major progress in recent years. Thin-film solid electrolytes in the range of several nanometers up to several micrometers have been prepared successfully. The most important reason for the development of thin-film electrolytes is the reduction in the ionic resistance, but there is also the advantage of the formation of amorphous materials with stoichiometries which cannot be achieved by conventional techniques of forming crystalline compounds. It has often been observed that thin-film electrolytes produced by vacuum evaporation or sputtering provide a struc-... 

Figure 8.12 (a) Nyquist plot obtained for the all-solid-state cell, ITOAVO3/PEO-H3PO4/ ITO(H) at 8°C, with the electrolyte being unplasticized. The WO3 layer was 0.3 pm in thickness (as gauged during vacuum evaporation with a thin-film monitor), while the electrolyte thickness was 0.24 mm (achieved by using 0.3 mm spacers of inert plastic placed between the two ITO electrodes), (b) Schematic representation of the equivalent circuit for this cell. 

Gratzel and co-workers reported an N3-dye-sensitized nanocrystalline Ti02 solar cell using a hole-transport material such as 2,2, 7,7 -tetrakis (N, N-di-p-methoxyphenyl-amine) 9,9 -spirobifluorene (OMeTAD) as shown in Fig. 17, as a solid electrolyte [143]. OMeTAD was spin-coated on the surface of the N3 dye/Ti02 electrode and then Au was deposited by vacuum evaporation as the counterelectrode. The cell efficiency was 0.7% under 9.4 mW/cm2 irradiation, and 3.18 mA/cm2 of Jsc was obtained under AM 1.5 (100 mW/cm2) [143]. The maximum IPCE was 33% at 520 nm. The rate for electron injection from OMeTAD into cations of N3 dyes has been estimated as 3 psec, which is faster than that of the I ion case [144]. 

The reaction mixture is transferred to a 2-L, round-bottomed flask with ethanol washing and the ethanol is removed by rotatory evaporation. Diethyl ether (1 L) is added to precipitate the electrolyte salt, which is collected by filtration and washed with ether. The crude electrolyte is obtained as a white solid (32-32.5 g, theory 34.1 g). The filtrate and washings were combined and evaporated to give a viscous brown oil, which was vacuum distilled through a short Vigreux column (15 x 2.5 cm). After a forerun of 70 mL of material boiling below 150°C (0.15 mm) the product (92-96 g, 53-56%), bp 150-155°C (0.1 mm), is collected (Notes 8 and 9). The forerun contained diethyl maleate, diethyl fumarate, diethyl succinate, and diethyl ethoxy succinate. The product is a mixture of diastereomers on standing some meso isomer, mp 74-75°C, crystallizes. 

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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