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Article Dans Une Revue Electrochemistry Communications Année : 2002

Final comment on reply to "comment on the electrochemical reduction of sulphur in dimethylformamide"

Résumé

In order to put a final point to this discussion ([1 and 2] and preceding paper), it is meaningless to debate on minor points (ECE or EEC mechanism, nature of the working electrode or choice of the supporting electrolyte). The major issue is that the work of Pletcher and coworkers [1] ignores the role of the electroactivity of S3− in sulphur and polysulphide solutions, which is now admitted for many years. The electrochemical behaviour of S3− is unquestionable (Li+ electrolyte or not; Pt or vitreous carbon electrode): in Li2S6+DMF solutions, only S3− and its dimer S62− are present at equilibrium [3] and the experimental voltammograms have clearly shown that S3− is the first reducible species (a “nice” reversible one-electron process) observed in negative direction in these solutions [4]. This reduction corresponds precisely (same redox potential) to the second step of the electroreduction of S8 and, obviously, to those observed by Pletcher et al. (peak III/IV) in S8+DMF solutions. Now, these authors assign this reduction to S4−, originating on the dissociation of S82−, without direct proofs and they conclude that S4− is a transient species. First, assigning a reversible reduction, observed by cyclic voltammetry between 248 and 323 K at 0.1 V/s, to a transient species (here S4−) is, in our opinion, unsatisfactory. This interpretation has not been justified by a scan rate dependence: only a temperature dependence (at 0.1 V/s) has been observed [1] in order to provide conclusive evidence (already shown in the past [4]) of a homogeneous chemical process. Second, the mechanism proposed by Pletcher et al. is simple but it does not contribute to the interpretation of the electrochemical behaviour of polysulphide solutions. In fact, the crucial question is: what is the chemical behaviour of S82− on the timescale of cyclic voltammetry? A fast disproportionation or a fast dissociation? A dissociation of S82− seems, at first sight, to be reasonable, but S4− has never been observed by time-resolved spectroelectrochemistry (at several scan rates (between 1 mV/s and 1 V/s) and at several temperatures (between 223 and 313 K) under semi-infinite or finite diffusion conditions) and, unfortunately, such a reaction does not account for the fast variations of S3− and S32− during the second reduction step of S8 [4]. Consequently, a disproportionation of S82− appears to be more attractive in order to understand the key role of S3− but, its kinetics is fast on the timescale of cyclic voltammetry. A possible reaction (widely discussed in the past [5]) could be: (1)S82− S2+S62− (acleavagereaction) Therefore, the electroactivity of S3− is simply described by these two reactions: (2)S62− 2S3− (adissociationreaction) (3)S3−+e− S32− (reductionofS3−) Finally, the transient species should be S2 (rapidly converted into S8), rather than S4−.

Dates et versions

hal-00277892 , version 1 (07-05-2008)

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P. Leghié, J.P. Lelieur, E. Levillain. Final comment on reply to "comment on the electrochemical reduction of sulphur in dimethylformamide". Electrochemistry Communications, 2002, 4, pp.628. ⟨10.1016/S1388-2481(02)00395-8⟩. ⟨hal-00277892⟩
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