Polysulphide stability

The presence of sodium hexamethylene bis-thiosulphate results in the formation of hybrid polysulphidic/hexamethylene bonds. The polysulphidic portion of these hybrid bonds reverts to monosulphidic during reversion but stability is achieved by the presence of the hexamethylene. 

Photocorrosion can be prevented by adding a redox couple to the electrolyte whose potential is more favourable than the decomposition potential such that the redox reaction occurs preferentially. When n-CdS is used as photoanode in aqueous electrolytes, the electrode is photocorroded since the reaction, CdS -1- 2h - S -1- Cd, occurs readily. By adding NaOH and sodium polysuphide to the electrolyte (Ellis et al, 1976), photocorrosion is prevented. The /S redox couple preferentially scavenges the photoholes. At the anode, sulphide is oxidized to polysulphide (free sulphur) and free sulphur is reduced back at the dark cathode. Similarly n-Si anodes have been stabilized by using a nonaqueous electrolyte containing a ferricinium/ferrocene redox couple (Legg et al, 1977 Chao et al, 1983). Unfortunately, a similar stabilization technique cannot be applied to photoelectrolysis cells. Some examples of electrode... 

The bimolecular nature of sulphur trioxide is cited in support of these views, and the absence of colour in the polythionic acids is mentioned as being opposed to the polysulphide linking of some of the earlier formulae. Nevertheless, the above formulae do not appear to be in agreement with the relative stabilities of the acids, but suggest that the tetrathionic acid is less stable than the more compact trithionic acid, or than the more saturated pentathionic acid (cf. pp. 211, 215). 

The monosulphide and polysulphides are formed by burning the metals in sulphur vapour, by the action of sulphur on the metals dissolved in liquid ammonia, and by the action of the molten metals on sulphur dissolved in toluene. Hydrates or alcoholates and, in some cases, the anhydrous compounds may be prepared by dissolving sulphur in hot solutions of the hydrosulphides or monosulphides. Potassium, rubidium and caesium give all the sulphides where = 1, 2, 3, 4, 5, or 6 sodium only up to the pentasulphide, and lithium only those for which x = 1, 2 and 4 (Pearson and Robinson, 1931). All the metals form two polysulphides of relatively outstanding stability one is invariably the disulphide, and the other tetrasulphide in the case of lithium or sodium, and pentasulphide in the case of potassium, rubidium or caesium. The amount of water of crystallisation and the solubility decrease with increase in atomic number of the metal, the gradation being most marked between sodium and potassium. 

Unlike the case of the regenerative PEC system, sulphur formed at the photoanode (and dissolved as polysulphide species, S( c+i) ) is here not balanced by the reduction reaction at the counter electrode, because of the simultaneous reduction process taking place at the storage electrode. As a result, sulphur is accumulated in the photoelectrode compartment, and is removed only in the subsequent discharge process. This dynamic variation in electrolyte composition may have a profound influence on the stability of the photoelectrode and electrolyte, and on cell potential. To minimise these effects, either excess polysulphide must be included in the photocompartment, or a limit must be set to the maximum depth of cell charge and discharge. 

An example of a two-electrode PECS that does not need a membrane is polycrystaUine Cd(Se,Te) in an alkaline Sn VSn electrolyte (G. Hodes, unpublished results). Menezes et al. (1977) reported that acidic Sn VSn" electrolyte stabilised CdTe photoelectrodes to some extent. In experiments using polycrystalline Cd(Se,Te) on an exposed (on the back side) Ti substrate, this electrolyte was found to stabilise the photoanode somewhat, but not very efficiently. However, use of alkaline Sn VSn" resulted in considerably improved stability (although still inferior to polysulphide). On illumination, metallic Sn deposited on the exposed backside (the Ti substrate) of the photoelectrode. In the dark, this Sn discharged (current flow in the... 

JI 11] Jl X., Evers S., Black R., et al, Stabilizing lithium-sulphur cathodes using polysulphide reservoirs , Nature Commun, vol. 2, p. 325, 2011. 

Obviously, radicals of the type R-S cannot be stabilized in this way. It may therefore be predicted that the ease of homolytic cleavage of polysulphide links is in the order disulphide < trisulphide < tetrasulphide — higher sulphides, since the following radicals are involved ... 

The reaction of sodium polysulphide with dihalides containing 4-5 carbon (or carbon + oxygen) atoms in the main chain may produce monosulphidic ring structures in addition to some linear polymer. This reflects the well-known stability of five- and six-membered ring structures and is illustrated in Table 15.2 (Berenbaum, 1%2). 

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