Electrochemistry in Supercritical Fluids

The earliest studies of electrochemistry (as opposed to corrosion) in SCFs were almost certainly carried out by Williams and Naiditch who, in 1970, described a two-electrode experiment in which they formed dendritic deposits of silver on a platinum electrode from dense gaseous solutions of AgN03 in ammonia [35]. As these experiments were carried out in sealed glass vessels only slightly (7 K) above the critical temperature for NH3, it is not certain that the fluid was actually in the supercritical (sc) - as opposed to close 

One way to overcome this problem is to mix SCCO2 with a more polar cosolvent, such as methanol or acetonitrile. Although these mixtures will still form a single-phase SCF 

When using SCFs - and particularly when using mixed-solvent systems - it is important to characterize the phase behavior of the system since the addition of significant concentrations ( 1 mM) of electrolyte are likely to alter the phase behavior. This can be achieved conveniently by using a view cell in which the different phases can be characterized as a function of temperature, pressure, and composition [53]. 

An alternative approach to the use of a cosolvent is to start with a fluid with a higher dielectric constant. Hydrofluorcarbons (HFCs) have been used as supercritical solvents for electrochemistry as they are polar and give higher dielectric fluids whilst retaining 

25 pm-diameter Pt disk electrode scan rate 20 mV s . For clarity, only the forward sweep of each voltammogram is displayed, with the ferrocene wave recorded first and the cobaltocenium wave immediately after, each wave originating from -0.20 V. Reprinted with permission from Ref [54]. Copyright 1994, American Chemical Society. 

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Other low-temperature studies have been motivated by the desire to characterize and understand processes occurring in unusual media. For example, the use of liquid ammonia [8-10] and liquid sulfur dioxide [11-13] naturally requires reduced temperatures unless high pressures are used, as is done for electrochemistry in supercritical fluids [14]. Frozen media are interesting systems in terms of mass transport phenomena and microstructural effects. Examples include glasses of acetonitrile and acetone [15], frozen dimethyl sulfoxide solutions [16,17], and the solid electrolyte HC104 5.5 H20 [18-20]. 

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