E. Levillain

Visible time-resolved spectroelectrochemistry : application to study of the reduction of sulfur (S8) in dimethylformamide

Abstract We have studied the reduction of sulfur in dimethylformamide (DMF) by time-resolved spectroelectrochemistry coupling electrochemical techniques (cyclic voltammetry and chronoamperometry) with intensified multichannel detection spectrophotometry. The measurement system allows up to 2000 spectra to be recorded in real time during the electrochemical perturbation and the derivative of absorbance to be calculated at all wavelengths of the visible range in order to compare electrochemical and spectroscopic measurements. The comparison between experimental data obtained under semi-infinite linear diffusion conditions and under thin layer conditions, between −40 and +60°C, shows that S 8 is reduced to S 2− 8 and that the disproportionation of S 2− 8 is weak but leads rapidly to the formation of a low concentration of S − 3 . We show that the reduction prewave, observed at a potential just lower than that of the reduction of S 8 and assigned by various authors to acidic impurities, is well resolved under thin layer conditions and gives a high concentration of S − 3 near the working electrode. This reduction prewave is interpreted as the reduction of S 2− 8 and/or of its monomer S − 4 .

On the understanding of the reduction of sulfur (S8) in dimethylformamide (DMF)

Abstract We have re-examined, using cyclic voltammetry, the electrochemical reduction of sulfur in DMF. The main purpose of this work is the determination of the electrochemical mechanism of this reduction and the identification of the opening step of the ring S8. We propose the following electrochemical mechanism which neglects the weak disproportionation of S82−: S 8 c + e − ⇄ S 8 c - ( E c ) S 8 c - ⇄ S 8 1 - ( K 1 ) S 8 1 - + e − ⇄ S 8 1 2 - ( E 1 ) a n d E 1 > E c S 8 c - + S 8 1 - ⇄ S 8 c + S 8 1 2 - ( K 2 = exp u2061 [ ( F / R T ) ( E 1 − E c ) ] ) This mechanism (ECE) takes into account the ring character (c) of sulfur in the solution and the linear character (1) of the S82− polysulfide. It has been simulated and adjusted to the experimental data in a wide range of temperatures (233 to 313 K) and scan rates (50 to 2000mV s−1). We shall discuss the choice of this mechanism and the results.

Polysulfides in dimethylformamide: Only the radical anions S3− and S4− are reducible

Abstract The reduction mechanism of polysulfides in dimethylformamide (DMF) is examined. With this aim, Li 2 S 6 + DMF solutions have been investigated by using cyclic voltammetry and time resolved spectroelectrochemical experiments coupling spectrophotometry and cyclic voltammetry. It is shown that the only reducible polysulfides in these solutions are S 3 − and S 4 − . The experimental results are interpreted by the following mechanism: This mechanism has been simulated and adjusted to the experimental voltammograms in a wide temperature (233 to 313 K) and scan rate (50 to 2000 mV/s) range. We discuss the experimental data, the choice of the mechanism and its ability to describe the experimental data. It is shown that this mechanism also describes the second reduction wave observed in solutions of sulfur (S 8 ) in DMF.

Polysulphides in dimethylformamide: a micro‐Raman spectroelectrochemical study

The reduction of sulphur and the redox properties of polysulphide anions in dimethylformamide (DMF) were studied by time-resolved spectroelectrochemistry, coupling cyclic voltammetry with micro-Raman spectrometry. The measurement system allows one to record several spectra in real time during the potential scan of the working electrode. The data obtained under thin-layer conditions, at room temperature, were compared with those obtained by cyclic voltammetry and by visible time-resolved spectrophotometry. The variations of the Raman (535 cm-1) and visible (610 nm) characteristics of the radical anion S3- during the potential scan for S8–DMF and Li2S6–DMF solutions are presented and these results are discussed.

Electrochemical impedance of solutions of polysulfides in liquid ammonia: experimental evidence for the Gerischer impedance

Abstract It is shown that the electrochemical impedances measured in solutions of polysulfides in liquid ammonia (lithium polysulfides Li2Snue5f8NH3, ammonium polysulfides (NH4)2S4ue5f8NH3, and mixed tetrasulfides (NH4)xLi2 − xS4ue5f8NH3) measured at the equilibrium potential on a stationary gold disk electrode can be analyzed in terms of the Gerischer impedance, i.e. the impedance predicted by Gerischer when the electronic charge transfer is coupled to one (or several) chemical equilibria in solution. Our experimental data are believed to be the first unambiguous experimental evidence for this type of impedance. The comparison of experimental impedances obtained for gold and platinum electrodes confirms that we are dealing with Gerischer impedances. In a given solution, several Gerischer impedances can be observed in different frequency ranges. The theoretical analysis leading to this impedance profile is recalled, and the experimental conditions to obtain valuable data are given. Finally, the data analysis is detailed. The experimental data are weighted by the modulus of the impedance. The high frequency Gerischer impedance originates from rearrangement of the S2−3 polysulfide involving the ammonium ion. The possible origins of the low frequency Gerischer impedances are discussed.

Electrochemical and spectroelectrochemical study of the oxidation of S2−4 and S2−6 ions in liquid ammonia

Abstract The electrochemical study of solutions of lithium polysulfides (Li 2 S n ) or ammonium polysulfides ((NH 4 ) 2 S n ) in liquid ammonia allows the specific study of the oxidation of S 2− 4 and S 2− 6 ions. This has been conducted by using cyclic voltammetric experiments and, for the first time in liquid ammonia, time resolved spectroelectrochemical measurements coupling cyclic voltammetric and absorption spectra recording. These experiments have been performed vs. concentration, temperature and the stoichiometry n of the solutions in order to determine the oxidation mechanisms. It is shown that the oxidation of S 2− 4 leads to the S 2− 6 and S − 3 polysulfides. The oxidation of S 2− 6 gives S 2− 8 , unstable in liquid ammonia. However, the lifetime of S 2− 8 allows it to be oxidized into S 8 , soluble in liquid ammonia. It is also shown that the solubilization process of S 8 in liquid ammonia is slow enough to allow the observation of the reduction of S 8 . The reduction wave of S 8 has the characteristics associated with an insoluble species. The spectroelectrochemical experiments also display the solubilization process of sulfur (S 8 ) in liquid ammonia and give evidence for the formation of S 4 N − species resulting from this solubilization. The oxidation processes of polysulfides S 2− 4 and S 2− 6 are discussed.

The S3− radical as a standard for ESR experiments

Abstract It is shown that the S3 radical anion, observed in equilibrium with the polysulfide S 6 2− in solutions of lithium hexasulfide in liquid ammonia (Li 2 S 6 −NH 3 ), can be used as a standard for ESR experiments. This standard is convenient for the study of broad ESR signals of the order of 100 G.

Reduction of S−3 and S2−6 polysulfide ions in liquid ammonia

Abstract In non-aqueous solvents, the S 2− 6 polysulfide ion is partly dissociated and is in equilibrium with the S − 3 radical anion. Several electrochemical studies have considered the reduction of these polysulfides. However, it has not been possible to distinguish the reduction of S − 3 from that of S 2− 6 . In the present work we give the results of cyclic voltammetric experiments performed for Li 2 S 6 -NH 3 solutions at various temperatures and scan rates. They show that, in liquid ammonia, S − 3 is more easily reducible that S 2− 6 . The experimental results lead to the conclusion that the kinetics of the equilibrium between S 2− 6 and S − 3 , which is strongly temperature dependent, play a key role in the reduction mechanism of S − 3 and S 2− 6 . The mechanism involves two one-electron transfer reactions leading to S 2− 3 and S 3− 6 , coupled to an irreversible decomposition reaction of S 3− 6 into S 2− 3 and S − 3 . The proposed mechanism has been confirmed by numerical simulations that give an estimate of the reaction rate constants.